M.,
We have let my son spend the night at my mom's (she just turned 53) house since very early on (after he passed the colic stage, lol). My mom has a crib and a bedroom for him and was preparing her house as I was still pregnant. We went away to Vegas when he was about 9-10 months old and he stayed with her. At this point, my mom takes Jacob (2 years old) once a month over-night. She picks him up from daycare and takes him there the next morning. I swear, it seems she lives for this time with her grandson....AND it's nice for my hubby and I to have a night together alone. I have absolutley no reservations when Jacob is with my mom. I miss him, but I'm totally confident and comfortable with the situation.
Then, there's my inlaws...they live in the area and Jacob has only been to their house 3 times - and we were with him every time. He won't ever spend the night or even be there alone as far as I can see it. I know that sounds mean, but it's my gut instinct with them. They have 2 dogs and one with a bite history (bitten at least 3 people). Yet, they argue about putting the dogs away if we come over for a visit. I'm not a dog-a-phobe, either. We have two German Shepherds and my mom has 2 Great Danes (well, she had to put one down last month).
As a last resort, I will ask them to babysit our son. I simply do not trust their judgements with regard to him. I'm 100 times more comfortable leaving him with my 15 year old sister than them. I won't go into the reasons why (there are a lot), but the bottom line is that I'm not comfortable with it. Enough said. If that's how you feel about your inlaws (for whatever reason), then you are entitled to feel that way.
In my opinion, you have a pretty easy out with them as you can say that you aren't comfortable with your little girl being over night yet. If you were letting your daughter spend the night elsewhere, that would be harder to explain why there and not with them - you know? Have you talked to your husband about it? What does he think? Does he agree with you? If you are in agreement, maybe he can say something to them, or you can both voice it together? Just an idea. I don't think your IL's are trying to do anything except give you a free night to spend with your husband, and that's sweet. But, if you aren't comfortable with it, that's 100% fine.
Good luck,
T.