Not my own mom or sis, but I get where you're coming from.
The more wholesome, loving, kind and innocent she is, the hotter the contrast is to see her getting absolutely railed in front I'd those she loves. Especially if her screams for help, muffled by a cock brutally trying to fuck the voice out if her, get ignored and she has to watch as the people she love most of the world, the son she almost died bringing into the world or her protective father, smile and openly start to jerk off to it.
Idk if them being YOUR loved one makes it especially hot for you, but I do get the urge every time I see an innocent girl who looks like she'd blush just from hand holding, or a wholesome pta mom busily Cleaning up after her kids with a happy, loving smile that could light up a room.
I get that. I find contrasts/contradictions in characters like that really hot too.
I really get turned on by the idea of the rapist having a character like a perfect kid next door, with a clean, athletic look and youthful smile. And then he and his equally clean-looking buddies from school tie somebody down and take out massive cocks and absolutely ruthlessly plowing into somebody and tear them apart while their friendly smiles and character never slip. The idea of the person they do it to being also wholesome and sweet is also hot; and she cannot believe these sweet guys are humiliating her like this.