my kids have never slept at my inlaws without us. They have babysat (once we got them all to sleep) but never had them for more than an hour or 2.
My dad on the other hand has had my kids since the eldest was as young as a few weeks old. I FF my DD1 and as my mum died when she was 2 weeks old, we all used DD1 as a bit of a support at such an awful time.
My dad looked after DD1 when I went into hospital when I had my appendix out (DD1 was 9mnths old) and he even took her on holiday at 13mnths old so me and DH could have some time out (was pregnant with DD2) I had 10 child-free days and it was such a nice break - I missed out on DD1 starting to walk, but little sacrifices!
I dont think my dad has ever had DS to sleep over without us though, but up until April we only lived 10 mins away so there was no need. Dad would come over and babysit and then stay over with us.
We now live 700 miles away from anyone that could babysit or have my kids overnight, so it isnt really an issue, but my inlaws and my dad are both visiting within the next 2 months and me and DH are looking forward to getting out for a few evenings!!!
It needs to be personal choice. The first time I left DD1 with anyone was the night my mum died, and I left her with my now SIL. It was only for about 4hrs, but it was long enough.