Hi, GolfingGirl. I also have a younger sister, though we have a bit more of an age gap of four years (ending up five years apart in school). I still vividly remember the night she excitedly yanked me into her room to tell me all about losing her v-card, how she'd been excited about this for years, and how disappointing and not great it was but how she was still excited to have done it. I think she was about 16 and me 20 when it happened. I had a lot of conflicted feelings about what she was telling me.
On one hand, I didn't have a libido or sexual attraction and had been sex repulsed most of my life, so I really couldn't relate to or understand her urges or excitement at all. As well, I had just gotten out of an... well, let's just say very toxic relationship with... nonconsensual experiences, had not told anyone in my life what really happened, and was a little afraid of my sister's enthusiasm for sex and afraid that the same thing that happened to me might happen to her. I wanted to protect her from that so bad. On the other hand, being a libidoless asexual and not understanding those feelings at all, I'd done a lot of research and giving myself a thorough sex education trying to understand how other people felt, and wanted to tell her all the ways she was allowed to stand up for herself and ask for her own pleasure and how sex could be good and it didn't have to be boring or disappointing. But she also knew I was ace, and I was, admittedly and kind of shamefully, embarrassed to tell her everything I wanted to tell her for fear she might think I wasn't really ace or something. I also didn't want to reveal too much and reveal the bad things that had happened to me.
But I also doubted that what she wanted or needed in that moment was some lecture or lesson from me. So, I simply took my cues from her attitude and did what she wanted from me: I listened and I didn't judge. I let her tell me all about her experience, I asked a few questions, I told her I was happy that she was overall satisfied with what happened and that I hoped it'd be better in the future with more experience.
And you know what? It worked out. Because my sister is her own person. She was young. She's grown tremendously since then. She's educated herself way more. She learned from her experiences. She has more confidence and has more practice asking for what she wants and upholding her boundaries. She's exploring her sexuality in the ways she wants to, and in doing so, she's figuring things out way better than she would have if I had just tried to tell these things to her.
It sounds like you're carrying a lot of responsibility around for your sister, her feelings, and her experiences, and it sounds like this has been extremely unfairly put on your shoulders by your mom and maybe a little bit by your sister. Try not to forget that, in the end, your sister is her own person. She's gotta make her own mistakes and have her own experiences and go through her own growth. You had some experiences with sex you haven't 100% enjoyed and seem to be struggling with a little bit of needless shame over. But you learned from those experiences, right? You now know better than you did. Your sister is capable of doing the same thing! Even though this pressure and responsibility has been put on you to take care of your sister, she's her own person, and she's growing up. Not only is she going to make her own mistakes and do her own thing and take her own path - she needs to do this. As a younger sister, she probably really wants to do this. Older siblings can often struggle with the accountability they feel over their siblings, but younger siblings can often struggle with feeling like they're living in the shadow of their older sibling(s) and never getting to do anything first. So try to do some work in letting go of the responsibility you feel over the path your sister takes. She's as capable as you are - and if she's struggling, she knows she has you to come to for support/advice/comfort/help.
Also, you're 17. You're nearing an age where you might be thinking about leaving for college or potentially getting a job and living with some friends. You can't be there to always be watching over her (and she likely doesn't want you to be). My mom and sister had a really rough relationship with each other, and I was often the buffer for them and the interpreter between them. Going to college was scary because I seriously wondered if they might kill each other without me there to balance them out. But you know what? That also worked out. Without me there, my mom and sister had to learn to live with each other without me as a buffer between them. Plus, my sister was getting older and growing out of some of the hormonal teenage acting out. They slowly learned to live more peacefully with each other and eventually grew to have a close relationship.
So, it might help for you to think about the fact that you're doing some moving on in your own life, and it's good to focus on the things you're going to want to pursue and experience and go after. You're allowed to live your own life. You can't hold back your own life in order to try and protect your sister. Just like she's gotta make her own mistakes and figure out for herself what she wants, you've got to keep doing this for yourself. Maybe instead of focusing on your sister's first time, you could spend some time re-evaluating your own desires, boundaries, and wants. Do you feel you have a good handle on what it is you like/want? Maybe you want to focus on a break from sexual encounters right now and spend time on yourself. Maybe this is time for yourself is masturbation and exploration or maybe it's re-connecting with an old hobby or getting into a new one you've been interested in. I'm not sure how close to graduation you are, but maybe this is just time to dream your biggest dreams about what you want to do after graduation.
I do think your idea of a secret gift exchange between you two of condoms/lube is smart and thoughtful, though. It opens the door to let her know that you're there for her, discuss some boundaries regarding your shared bedroom, and stays more within your comfort zone. As well, since this has been weighing on your mind so much, doing this may help you feel as though the situation has been adequately addressed and you can spend less time worrying about it. I also want to confirm that you do not have to share your experiences with your sister if you don't want/aren't ready/are uncomfortable with doing so. It's okay to have experiences that are just yours for you alone. They do not have to be a lesson for your sister.
I think the best thing for you to focus on right now is yourself rather than your sister. You said you didn't really have regrets about having sex, but I'm hearing a whole lot of shame and self-judgement about your experiences. Your sex story is not ugly. Your sex life is not ugly. You are allowed to have your own experiences. Things will not always go perfect or how you want or exactly as planned. That doesn't mean you need to put yourself down for this! I think it's time to do some work in letting go of the feelings you have and moving forward from them. Try spending some time re-framing your thoughts. Instead of "my sex life is ugly because I lost my virginity very rushed in an uncomfortable car" try re-framing it as "having sex for the first time - nor any of the times after - doesn't define me, nor does it have to define any potential future experiences. I learned that I don't really want to have sex in a car again, nor do I want to do it in a more spontaneous manner when there may not be time to really enjoy myself. Now I know, and if/when I pursue this again, I know more about what I want and will be able to advocate for that. One less than great experience doesn't make me bad or ugly. My story is made up of all sorts of highs and lows of all sorts of intensities. That is life. I will have more bad times, but I will also have great times. And that's all okay." Does this sound like something you could do?