How to tell my MIL that I dont want my DD leaving me overnight yet.

[OceanPrincess]

Reet whether you are way over protective or not, that is your right as her mummy. You get to decide

Anyway, you are well within your rights to not want her to go what sounds like quite far away. A plan ride? Would she be with someone? She is far too little to be an unaccompanied minor, isn't she? I'm not sure, tbh. But for me, a plane ride is a big deal! I wouldn't want to be shuttling my kids off on a plane without me! And I am quite happy for my LOs to have sleep overs with my ILs... But not for that long, and not that far away.

I think you just need to tell them, nice and clear, "I am not comfortable sending DD away for that long. I'll let you know when I'm ready for her to have an adventure with you guys. At the moment I just think she is too young. Thanks for your offer."

[onthefly]

she really shouldn't need any reason beyond that you don't want her to do it. Period.
I agree with OP, just keep saying you don't want it. I would suggest don't make excuses, that just gives her something to argue about; just tell her no.
Also, I don't think you're weird. Some parents are ok with this even from a young age, personally, I'm not. My kids started have 1-night sleepovers once they were 2+ and sleeping well. They've never stayed away more than three nights total and DS is now 5.5