It’s evident you are desperate for a break, it’s however not a given that your in-laws will be there ready and waiting for you to dump your dear children and run away with the engine still running
I think it’s OK to ask for the odd baby sitting session, but less so to expect retired parents or in-laws to take on small kids for a weekend if not willing to.
Some do and love it, some want to but are too tired or simply don’t want to. It’s really lovely when it works out, but it’s a bonus not to be demanded.
For me sadly, I don’t even bother asking my MIL to baby sit as she’s literally causes me more work than help .
It becomes a weekend of me cooking for her and entertaining the next day, making bed and cleaning bedsheets just for an evening of baby sitting… rather pay a sitter and be done !
She doesn’t manage the kids well… I don’t blame her , I just don’t ask her often as not really worth it .