C.B.
mine has been staying over at my mom's, usually once or twice a month, since about 6 weeks old (whenever he started sleeping through the night, although i do think she had to get up with him some times, here and there). i think it's a matter of your personal situation, but if you get them started on it young it is normal for them and won't be a problem. could you leave the 18 month old too, or is that too much for pop pop to handle? it would be easier if they were together, i bet. i do suggest, since your oldest is just now doing this for the first time, to perhaps stay at your MIL's, like you suggest. just in case. good luck! once you get started (IF she has a problem) and she can get over that hump, she will LOVE it and it will be some of her best childhood memories. good luck!
after reading the other responses - i am almost offended at the vehemence in some regarding NEVER letting kids spend the night with family. yikes. i guess if you saw your grandparents all the time and are super close to them, there's maybe no "need" to spend the night...but my son's grandma lives 30 minutes away, (okay- lately we do see her almost weekly!) and they have SUCH a tight bond, i would never deny either of them that. what on earth is wrong with being close to family like that?? i wonder if these people don't have family members they can trust? or do they think they as hardworking parents don't deserve a night off occasionally? or what? i am just baffled. i hope the judgements don't convince you never to do it. there is NOTHING wrong with a child being close to his or her grandparents. family is everything! heck, half the time i go out to my mom's with my son just for the pure enjoyment of it! but it is nice for her to have him all to herself sometimes too. they adore each other. i just can't get over that some parents don't think that's okay.