Have nc for this in case people I know read this. A lot of infomation might not be relevant but thought it better to get as much background down as possible, not to drip feed etc.
OK, Dh, twins (age 4) and I are from UK but live in Germany and have done since twins were 1. Dh has a relative with 3 dc similar ages to our twins. We do not go back to our home town very often but mil and fil have visited us many times here (they are separated so dont come out together) : mil visits every 2 months or so and stayed with us for 15 days (!) over Christmas. I get on fine with mil, though a visit of few days is generally enough for me. Our twins are the only gc of my inlaws. The children of dh relative are the only gc of his and her parents.
As we live overseas our twins dont see much of their cousins so dh and I thought it would be good to get the 5 little people together for a holiday so they could spend time together. We booked a caravan on the beach near where we are from (and where most inlaws still live) so that cousins could all play etc and family could "pop in" for a while if they wanted to. All very exciting.
Mil, sil & partner, mother of relative (aunty of dh) & her husband and another aunt have all now booked into the same caravan park for the whole duration of our holiday. They all have somewhat forceful personalities. So finally getting to the point aibu to feel my holiday is being taken over and turned into something else?
This is the first holiday dh, twins and I have been on as a family, and its not much being a caravan holiday in April in the north east 
but it was our holiday. Now I just feel its being taken over, inlaws to tend to take over any gathering and I just feel trampled over. Noone asked how we felt about them joining us, they just booked it and presented it as a fait accompli.
Am only mentioning inlaws as my parents are both dead (just incase anyone would wonder how I would react if it was my parents/what they will be doing).
Apologies for the length, hope it makes sense!