What I don't like about incest is that motherly feelings don't work this way. A real mother wouldn't have tested the formula on her son and even if did, she would have dedicated all her efforts to fix him - and definitely not play with her son.. especially sexually.

Now, I don't care about Your deviations, but it simply unreliable - and if You wanted casual careless fun, then why not make her shrink some lab assistant? Plenty of choices there - and surely she would play with him and so on.

I also don't like the contrast of their behaviours in the two chapters. While one can barely understand Jake's switch from scared to unwilling - but I personally believe it would take hours or even days to get accustomed to such situation for a person that never was into such fetishes (and I understand he wasn't?) - but the Amy's turn from motherly love to forceful shrinking... I think it would be much better if You gave Jask some room for decision or at least explained her emotions transition some more.

I could find it believable that she would like to have some playtime once discovering the antidote - and he can always shrink back later. It would also convince Jake that he doesn't have to worry that much.

But I'm not trying to tell You how to write your story, just.. please don't make the characters so ridiculously shallow and unreliable like so many other stories (both Your and other authors).

I can see some potential here (that's why 4), it would be a pity if they turned to be mere puppets hanging off a deus ex machina driven plot.