i can't remember when i first left my DS overnight but i'd say he was between 12-18 months. he'd been waking every 45min-2 hours every night since he was born so one night i slept at my MIL's house while she & DP stayed here with DS. we've left him a few times overnight at her place since but she lives round the corner & he is actually excited to go there to stay (he's 2 & 7 months). i wouldn't have felt comfortable with it until i was absolutely sure that he was happy to be there & that i know DP's mum has very similar ideas around parenting/comforting as we do.

it is lovely that she wants to help, but i agree with ladybirdflies that there are far better ways she could be helping at this stage, especially as you are BFing & essentially wouldn't be getting extra sleep as you'd still be expressing when baby would have been feeding anyways!

there are lots of years to come when she can babysit overnight (if you're comfortable with it)