First of all, I'm so sorry that your sister abused you, and that you have been told that women can't rape other women: that's absolutely untrue.
In regards to virginity, your mom was sort of on the right track when she said it didn't matter; virginity isn't a physical state or something that has a medical definition, and it's something everyone gets to define for themselves. But, whether it matters is also something that people get to decide for themselves. If it feels important to you to have a concept of virginity and a way to define yourself within that concept, then it's important.
That said, I think most people would agree that virginity - however they choose to define it - is about sex, and abuse is not sex. Sex involves two people who are consenting throughout, and abuse or rape does not fit that definition, so when you're thinking about all of this that's something to consider. We have a couple of articles that might be helpful too as you think about this:
20 Questions About Virginity: Scarleteen Interviews Hanne Blank
What's Sex?
I also want to check in with you about whether or not you've ever gotten any counselling or therapy around the abuse: it's not necessary, but it's something that many abuse survivors find helpful, and if you haven't connected with a counsellor or therapist but would like to we can have a look to see what might be in your area.