I actually made a post about my sister on a different account (I got confused logging in lmao and accidentally made a new account idfk lmao-) where I asked about coming out to my sister, she's over 20 years older than me and I'm still a young teen, she also has two kids (my nephews) (who I'll call M (two years younger than me) and J (a year older than me)) I think I replied to a few comments on the post saying I cam out to her and she was very supportive, said I could come out to her kids on my own terms, and I even told her about stonewall (I was wearing a stonewall sweatshirt) which is slightly important to remember.
Recently I moved to the same city as her, meaning id get to see my nephews more often. I've mainly been closer with M than J, mainly cuz we're both afab and trans, and before we both realized we're trans, we had very similar interests. Same now that we've both figured it out (also M hasn't specified what labels they use so apologies for the weird wording) Recently M came out to me when I visited them, and they eventually told me about how transphobic their parents are. His parents won't let him have pride flags and shuts him down so often that he no longer brings it up. Which, I'm no expert, but that's effed up. They've told me that my sister has said "Look at straight people! We don't have pride! We don't make our sexuality a big deal!" (Remember me telling her *in detail* about stonewall? like wtf-) and I recently found out that my sister made a comment about my transition, I don't remember exactly but it was along the lines of "Charlie isn't my kid, I don't get to make decisions about what they do", which made me so uncomfortable and angry. My sister is usually very friendly and upbeat around me but I never hug her back or anything, she makes me so upset that she treats her kid like this and the fact that she acts all nice and accepting to my face makes rude and insensitive comments about me in private. I'm just upset, and I want to say something mainly about her comments about me because they make me so uncomfortable, it's not something I can just shake off like this was my breaking point almost. My nephew M and I are so similar, our identities are similar (both gender identities and sexualities), we realized around the same age, use similar pronouns and it just makes me so mad that my sister would treat us like this. (Although It's not surprising considering our dad's views on trans and LGBTQ people) It's just impossible to shake off and I don't know what to do or how to handle this
I know this is late but...
I'm so sorry that you found this out about your sister. It's always worse when people act nice to your face and talk behind your back. At least M and you (I'm not sure how J factors into this) have each other, and you now know what's going on.
Please know that you aren't alone, you are loved, both you and M and anyone else who reads this is deserving of acceptance and it's rotten that your sister is this transphobic.
Some advice:
First of all, there isn't much that M can do, as M is your sister's child and neither of you are yet eighteen. I would advise that you get away from your sister, but I believe that your support is quite valuable to M. If you have any accepting family members, or friends, I would suggest that you talk to them (if you can, of course).
Sending you lots of good wishes and hugs (if you want them).