Special Needs Brother

[halongue]

I'm looking for a story with the following details:

• POV character is a young woman who is staying with her boyfriends family
• The boyfriends younger brother has some sort of developmental challenges, and so needs to be looked after
• The brother likes the woman a lot and they start to become sexual
• I think the brother is encouraged(?) to force himself on his mother, maybe?

I didn't read the whole thing, and haven't been able to find it despite an hour or so searching. Thanks for any help you folks might have!