Will try to keep this brief:
At Xmas my MIL said she wanted to take us all (her three DC plus partners and our DD - 18 months) to go on hols together for 1 week in August - it's FIL's 60th. I said OK but I wouldn't be around for the whole week as it is also a significant date in my family so my sister would be visiting from overseas. MIL said OK.
MIL has booked a house in France so my family and some of my sister's friends were also planning to book a house in France for the week, about 2 hours away from MIL's house. I plan to start the week with PILs on the Saturday then leave DH and DD with PILs on the Monday to join my family. Then my dad and I will spend all day Wednesday with PILs before going back to the house we have rented in the evening with DD where we will stay until Saturday. DH may come with us or may stay with his parents for the rest of the week.
When we mentioned this plan to MIL a couple of weeks ago she told DH how disappointed she is that we aren't staying the whole week (by 'we' read DD). She asked DH to tell me that my family would be welcome to stay with them for the week. This is a nice idea but it wouldn't work. My family can get on OK with PILs for a short amount of time but not a whole week. They are very different people. And it's the only holiday my sister and dad are getting this summer and they don't feel like spending it with my inlaws. Also, a couple of my sister's good friends are joining us.
I have now booked the house and I am in the process of emailing MIL to tell her about it and confirm plans. I'm a bit worried she'll go crying to DH. I really do think we've been really reasonable but would be interested to hear other's views. Thanks.