Threads along this theme on MN tend to divide between two basic lines. On the one hand there's the "I've never left my LO for more than a couple of hours and he's now 16" brigade.
And there's the "Oh... Timothy will be FINE! At the age of 2, I was independently rigging my own tent and camping in the wilds of the Serengeti" brigade.
Neither is wrong or right. And for open-ness and clarity, I'm more with the second than the first (although lower by several social classes!).
BUT what I would say is, if the primary care giver who's babysitting is, as you say, a health visitor, she'll have the two major's absolutely locked down (health and safeguarding) and she'll know all sorts about things like nutrition.
So, I'd say all the absolute bottom-line basics are covered.
And, as a relation, the love will also be there too.
If the worst happens (I'm not talking meningitis or abuse, because i'm assuming your MIL has 'got' those), and your LO cries all night and is inconsolable, then it won't kill either of them (they can both sleep the next day). Your LO won't remember or be scarred by it.
When your baby gets a bit bigger, there'll be loads of party/sleepover or playdate/sleepover invites, and some kids will manage those and LOVE them and some kids won't. My DD (who first stayed overnight with my parents at age 3 months) absolutely loved them. And felt REALLY sorry for the kids who couldn't manage them or who's parents couldn't allow it because of their hang ups.
There's an argument for the other side, but I kind of can't be bothered to make it, because someone who genuinely is there can make it for me, but better.