Perhaps you should tell your spouse to speak with your PIL pertaining this. Let them know that for once you wish to have just family time without them. Or plan a staycaytion/holiday without them knowing (if this is possible) It's like secretly planning without them following you. You can also let your spouse know to have the siblings to bring their own parents instead of you. If the next time they follow, you should be firm that the cost will be on them not you or spouse. We all need $ for savings too and to pay for someone else even family is a lot of $ too. Hope it helps one way or another.
Husband should take first crack at telling them. You can also say that you need personal travel time. Easier said than done I know but hopefully a straightforward talk helps
itsallabout_mytwokids