I'm confused but also kind of understand... so I think that his sister is trans and used to date the twins dad and maybe the dad gave her the money for the surgery...?
**spoilers**
Yup basically the dad was dating the brother and convinced him he would only love her as a woman so he paid for her surgery then after decided it was gross and dumped her. The twin always felt guilty which is why he is so nice to her younger brother.
And actually what made me mad is dat, WHY THE FUQ DID THE TWINS THOUGHT IT WOULD BE GREAT FOR HIM IF THEY DIDN'T TELL HIMHAHAHA FUNNEH.....
There were so many good screenshot moments in this especially when it had a minimalist style, so cute!