My friend won't stop calling me "sis," and I have other suspicions of him

It definitely sounds like there's a possibility he's trans. Of course he may be androgynous or agender as well. Maybe try to bring up trans people somehow and see what his reaction is. Or perhaps ask him if he likes his name. If not, that could be a big clue as to wether or not he's cis. Good luck, and I hope everything goes well!

 

Just 'cause he likes long hair and feminine things doesn't make him trans. Unless he comes out to you as transgender, it's not wise to be crossing your fingers about someone else's identity. He probably just falls outside of typical gender roles. (although, for your sake, I hope you meet someone who is trans so that you have a trans friend!)

As for him calling you gendered terms, have you tried telling him explicitly that it bothers you? Try not to beat around the bush, or he'll think you're joking. You just gotta say "hey, for real though, that bothers me, please stop." or something of that nature.

 

Having long hair and being feminine does NOT make a guy a trans girl.

I'm a trans guy, I'm growing my hair out to my shoulders. I also wear make up. I have no desire to be a girl, if I did, I would stay a girl.

As for the gendered terms, this has already been answered. Just tell him to stop.

 

Wow, he sounds a lot like me when I was in high school o_o I even affectionately referred to one of my friends as Onee-chan for a while... But that's unrelated! xP I used to do the same thing though, asking friends especially female ones if they thought I was girly or trying to make them see me as more feminine in subtle ways. though my hair was all the way down my back by high school.

Not that I'm suggesting that he is transgender but I'd say there is definitely evidence of that being a possibility here.

As for dealing with being called by wrong gender pronouns, we've all been there and its especially hard when you're in the closet. My parents have been affectionately calling me "Boychild" since I was little... And now even some of my friends will call me Boy because of that... Now I'd rather be called "Princess" personally but none of them know that and for now I just put up with it. So I know where you're coming from, and its not easy dealing with it. If you don't want to come out to him, then try to be gentle when you tell him you don't want to be called Sis or Onee-chan anymore, because he probably wont understand unless you come out to him. And tell him Senpai is still fine to reassure him if you're still okay with that

That's just my thought on it though, I'm not exactly an expert on dealing with this stuff since I just hide it, but that's just what I'd probably do in your case. ^^

Either way I wish you luck! ^^

 

Oh, its alright, I have other friends who DO call me things like Princess and other affectionate feminine things so that makes me feel a little bit better :3

Hehe! I'd love to trade that'd be nice wouldn't it?~ But I think the future is bright ^^ lil' bro~ ;3