My DS is 10 months old and I still wouldn't let him spend the night away from me. I don't think my DP would want him to spend the night away either.

We don't have parents near to us so I guess I'd never thought of overnights before but at 5 weeks of age is it a little strange that your IL's would want him overnight anyway? Maybe it's just me but it's still so.....brand new, bubs wouldn't even be used to his own sleeping arrangements yet.