Sorry, this may be long..
Bit of background - dsis (30s) lives with parents. She and DM go on holidays together, dad does his own thing. Usually a beach holiday or something once a year.
DH and I usually do holidays just us and prefer to be more active. Last year we went to London for a few days with DM and dsis for her birthday.
This year we decided to invite the ILs on holiday with us. They haven't had a holiday for a few years and I don't think would have the confidence to go away by themselves (MIL has had health issues), also we enjoy the same type of holiday. We went to Cornwall for 2 weeks and had a lovely time.
So, got home for dsis to tell me that she and DM (read: dsis) are annoyed that we would choose to go on holiday with the ILs but not with them!!!