The only reason I keep reading this it's for David and Alice content so I hope there's development for them soon, about Aunt Sarah... I don't like her anymore, her body may be great but this version with Uncle Peter is so dumb and passive, I mean, makes sense with the plot since she clearly likes David more and that's why she often has the initiative with him but with Peter she feels like a worse version of Lana.
That is unfortunately one of the pitfalls of being human (consistent in adult fiction but also happens IRL sometimes), once one breaks a moral rule (married but sleeps with nephew) then it's a slippery slope when/if a an opportunity of a similar nature (uncle's "seemingly innocent" advances) arises, one tends to explore that avenue versus avoiding it. Hence the corruption trope and it being believable.
I do agree with you on how it's being played out in the Sarah-Peter-ship is not my

either, a little too aggressive (Peter having evil intentions from the get go) vs just being an extremely lucky horny old Uncle who happens to be at the right place at the right time with a MILF Niece who has her horny engine fired up by her own horny nephew and another horny female orbiting about as well. Must be something in the water.
Would have definitely enjoyed it more with Peter mind being blown like David's in the bathroom with Sarah offering an "innocent relief-job", then getting wicked horny thoughts on how far he could push it.

But I do know there is an audience who enjoy it better the way it went down too. Just means I have a couple of speech/thought bubbles to re-write for maximum enjoyment.