I had my first night away from DS when he was about 16 months old, but I only left him with DH! MIL has started asking when she can have DS overnight and we've tentatively arranged it for this weekend. He's 21 months old. I'm comfortable with that now because he's sleeping well etc etc, but I DEFINITELY wouldn't have let DS sleep anywhere else at 5 weeks.

I can't believe you are being pressured to hand your baby over like a toy for your MIL to play with! I'd be letting both of them know in uncertain terms that neither of you are ready for that and to lay off the pressure. Maybe she can stay with you overnight and help out that way - take DS while you rest, help around the house etc. Or you could arrange a regular time for you to all go out/get together for coffee, do the grocery shopping together, for her to take him for a walk or look after him while you rest or do whatever you want to etc. I understand she wants to feel involved, so maybe you can work out another way for her to start bonding with her grandson... that doesn't involve her taking him overnight. I know MIL wanted to 'show DS off' to her friends and work colleagues when he was little - she wanted me to drive 40 mins so we could just 'pop' into her work!