No I won't accept that twist. First the friend doesn't wear a bowtie, which seems to be part of girl uniform. Second, they totally change the art at the end, more feminine compared to the beginning.
Did the sister give him a blowjob?
@raze2012 I think even an adult male won't help. In fact people had been making fun of her research before this boy. As mentioned in the early chapter, her research impose upon the trainee privacy too much.
Hmm so it seems like the sister doesn't personally like doing those stuff, but she just really wanted to prove her research. She looked disgusted doing all those things.