That's a bit harsh. The dev explained that he had to re-write the scene because it was drug-induced rape, which a lot of people didn't like. Now it's an alcohol-assisted bad decision by her, which is more acceptable to many.
I prefer this version, since I don't like rape, but I will agree with you that it still isn't fully believable. A hottie like mother probably is not so drunk or desperate that she'd agree to bang an ugly bastard like the uncle without some other contributing factor. To me it needs more development into how horny she is to explain why her body reacted positively to uncle's touches.
Personally, I might have gone with a story like she really wants to bang her boss (who was at the party), but she knows that's a career-ender / get her fired. So she drunk-rationalizes that a friends-with-benefits type bang with uncle might lower her frustration level and keep her from doing something she would regret at work.