I don't think even the PIL could actually get away with convincing themselves that she's some feckless hussy You'd be surprised Shouty - once an impression, no matter how erroneous, is formed it can be hard to break or shake.
I have encountered 40+ year old women with marriages of many years duration who have continued to be (and still are) regarded by their MILs as feckless hussys, but that is not the issue here.
Since his birth, PILS became accustomed to having their gs for 2 consecutive overnight stays per week which, not unnatually, has resulted in their forming a close bond with him. Nevertheless, at some point, the 2 night stay was reduced to 1 night in favour of the OP and her dh.
The OP's marriage is currently subject to considerable stress due to unresolved historic issues of honesty and trust and more recent issues that have arisen, one of which is the OP's observation that her dds are not as equally valued by their paternal gps as her dss.
The OP hopes that some resolution of this latter issue may be achieved by her dss ceasing his weekly overnight stay with his paternal gs in favour spending 2 consecutive nights per week with the OP and her dh (the child's father).
To this end, the OP's dh has been entrusted with the task of informing his dps that, with, effect from a week on Friday, they can no longer have their gs staying overnight with them.
I have serious misgivings as to whether the OP's plan is the best way forward for all concerned. In addition, I doubt that abrupt cessation of the overnight stays they have hitherto enjoyed with their gs will bring about the changes the OP desires with regard to her PIL's attitude to her dds.
From the information made available by the OP, I've attempted to provide her with additional insight as to how insecurities can manifest in ways that we are not consciously aware of and I trust that, in questioning her timing, she will accept that no malice was or is intended by my responses.
It goes without saying but nevertheless I'll add my voice to the throng that the OP's dh is a twat, but it would seem that he's the twat that she loves and wants in her life and I wish her luck in the unenviable task of persuading her Peter Pan to grow up.