"Op is not punishing the sil's parents by not doing stuff for them, the sil is by drawing the line in the sand.
I am not to assume she will help at any other time unless I have her full consent in advance.
Works both ways."
This doesn't make any sense.
They are not just SILs parents, they are the OP's husbands parents.
OP says she helps her husbands family because she likes helping people and was brought up that way. We don't know how much help they need or what sort of help it is.
It might be that they need total 24 hour care or it might be that she fetches them a pint of milk once a fortnight as she's calling in anyway. We don't know, but whatever it is, why are we assuming that the OP is doing it as a favour to SIL and SIL alone? They are her DH's parents too, just as much as they are SILs parents.
We also don't know if the SIL has ever said "OP can you help my parents for me please?" but I'd bet that she hasn't.
OP: Can you pick up this clarinet I've just bought?
SIL: Yes but next time can you check with me first?
OP: I will never help PILs again because of you!
It would be completely ridiculous for the OP to stop doing something she says she wants to do, just to spite SIL who has never asked her to do it, when it's not actually a favour to SIL in the first place, it's a favour to the PILs, the aunts and, at a push, OP's own DH.