OP
As somebody said once on MN “your lack of planning is not my emergency”
I had my kids later in life because (besides other reasons)I was squirrelling the money for childcare. 1000’s of £££’s.
When my last childminder moved out of the area I’ve changed my job pattern and work nightshifts only to accommodate ASD DS.
You are responsible only for your DC. That’s why you’re not out there in paid employment to accommodate your family needs.
School holidays are like birthdays and Christmas.They happen every year around the same time.Your SIL and BIL were well aware of that. They chose to fritter away their days off for the holidays banking on you having their kids.
Just NO.
They have 2 full time work incomes,they can pay for their fecking childcare without emotional blackmail and guilt tripping you, your DH and PIL’s.
If they start their 💩 at the table calmly ask them when did they help you out with your kids.Repeat like broken record as they will come with all sort of excuses.
You’re under no obligation.
Stand your ground.
People pleasing won’t take you anywhere (speaking from experience)