Time away totally depends upon the type of Grandparents and your relationship with them, which by the sounds of it is NOT grand.
I would never allow any of my children to sleep away from my husband and I anywhere until they could speak in coherent sentences and were able to tell me exactly what they did, who was there, etc...which is probably closer to about 5 yrs. of age. There were little neighborhood sleepovers up the street so I could tuck them in, or with their cousins.
Am I compensating for terrible things that happened to me when I was young? Absolutely. And frankly I don't care...My kids are safe and had healthy safe situations ALL the time.
Just say Sorry Grandma's and FIL and MIL, but mommies and daddies aren't up to being that far away for that long yet. I know it must be disappointing, but I just can't imagine being that far away yet.
Also, from the sounds of it, sounds like they'd try poisoning her mind while they had a chance....
Even your husband says he's not ready....so, as hard as it is to say, you've got to be your child's voice and haven.....Just say no....not now...