Okay, here's an unpopular opinion:
I'm not into moms, but i wish i had a sis. We'd fuck all day long.
And yes i don't deny it, i'd do it IRL as well.
You don't like it? Well i don't care, cry me a river.
Fact is, sister or mother they're just women in the end. Like any other women, so there is nothing 'disgusting' about it.
It's just that society brainwashed itself into thinking that it is.
Take protection seriously and you can fuck your moms and sisters all you want IRL as well, no problem.
And hell, if it's for example sister on sister then you don't even need protection, since chance to conceive between women is 0.