The question of when to wear makeup etc. is pretty tricky. I hate that this is true, but it IS the case that some people won't take trans people seriously if they present in a way that's associated with their assigned-at-birth gender. There are situations where you would probably feel safer, or more likely to be gendered correctly, if you held off on skirts or makeup.
I think, though, that it's fine to assess these situations on a case-by-case basis; you might not want to wear makeup when talking to a doctor about hormone access, for example, but maybe you have social spaces where you know it will be accepted and embraced by the people around you. You'll probably get a sense, at some point, for how you'll feel about presenting in different ways in different social spaces.
One of the things that can be tricky about being trans is having to weigh all these options carefully and sometimes making choices not based on what feels most comfortable and authentic, but what feels safe, or just likely to avoid questions or conversations you don't have the energy for on a given day. It's ok to make different choices around this at different times, or in different situations.
So: I get why your sister is encouraging you to wait a bit. I think it might make sense if you decide to do that in some settings! But I don't want to just tell you to hide a part of you that makes you happy, either. I wish I had a better and easier answer, here.