I just want to check.
So your parents booked the second stay without asking you the dates,
And then your sister and fam booked THEIR stay without asking you.
And so now you are locked in to those dates because everyone has booked their tickets.
Which happen to be the same as the dates of your partner's annual holidays when he wanted to go somewhere.
So, to avoid upsetting your parents and sister, and so that your parents and sister can have a nice holiday where you serve as host and hotel, you are essentially allowing your partner's annual holiday, after the awful Covid year, to be cancelled. And instead he will be made to host.
I think you need to contact your parents and sister and say, sorry about this, but you will have to change the dates of your second trip, perhaps to sometime in the fall, because you have mistakenly booked them to cover my partner's holidays, and he really needs a break and cannot be hosting guests at that time.
Yes it will cost them some money and inconvenience. That is their fault for booking BEFORE "asking" you (and since once they have booked, everyone sees the dates as unchangeable, they really are not asking you, they are just pretending to ask you).
Tell them next time they need to ask about dates BEFORE they spend any money or make commitments.