For context we are a family of 4 and DM lives on her own.
We've invited DM to our holiday once a few years ago, but prefer to do our own thing so tend to just go on holiday by ourselves.
DM goes in a huff whenever I tell her we've booked a holiday without inviting her. The cost would usually go up quite a lot as she's a 5th person and we normally have to either book a separate room or pay premium for booking a suite room to fit 5 in.
Aibu to not invite her to our holidays? We do spend some weekends and holidays and Christmas at either hers or she stays over at ours.