The MC defaults to "nice" and he tries to be accommodating, but he fairly frequently reaches his limit (and you can make him reach others sooner via your choices). But the only situation into which he sorta passively slides is dating Sandra. Everywhere else he has choices and makes them. (Save having a sexual relationship with Haley, I guess, but that was built into the game from the beginning.)
FWIW, I'm also on team Haley Was Lying when she said she fucks a lot. Constructing a justification requires a ludicrous amount of speculation and extrapolation, so we'll just have to wait and see.
On another note, I feel like a lot of games on here have a hard time giving MCs a personality. I guess it comes with being choice based and the fact that writing characters to have a real personality is hard as well as time consuming.
I think the MC has a fairly developed personality vs. the universe of erotic games/VNs/KNs, but the reason perspective characters are usually so bland is that people — definitely not me, but plenty of others — like to self-immerse, and giving the MC too many independent characteristics makes that difficult.
Actually they tease each other mercilessly. Of course, Haley is a world-class teaser, but the MC gets his own jibes in as well.
Isn't he jovially mocking her allegedly nonexistent ass in one of the very first scenes? You're right; Haley is merciless at times, but her brother gives it right back to her
unless he's legitimately angry or upset.
Haley and the MC love each other dearly, in every sense of that word, and they are extremely comfortable in dealing with each other. Neither one of them would deliberately do anything to hurt the other in any way.
It's still the best and most
real sibling relationship I've ever seen in one of these games, and sex has absolutely nothing to do with it.