Basically in Ch. 5, your 'rival' fucks his mom late at night. The scenes pretty tame, considering the mom is literally straight faced (intentionally). Besides that every other scene with him and your mom is avoidable (and the scenes you get for interfering are pretty hot).
BTW there are a few scenes where your trans cousin either fucks, gets sucked, or other activities with some of the other girls. Honestly, the amount of heat she's packing doesn't really make it ntr, more like two lesbians with a flesh strap-on.