My ex MIL was like this.
But even worse, my ex gave her a house key!
She'd let herself in whenever (to let the dog out, she'd say), then wander around commenting quite nastily on my terrible cleaning skills and my bad napping habits. I have fibromyalgia and would be floored by the fatigue sometimes.
I started leaving the key in the door, slightly turned, so she'd go around the house looking through windows and knocking them.
I'd lost my baby and had decided to miscarry naturally rather than have a D&C. I was in bed, in the depths of misery and pain and she came up to our bedroom demanding to know why I wasn't in hospital "saving" her grandchild.
A lock was installed on my bedroom door by my friend's husband the following day.
Exh was doing a run of nightshifts but she'd be down knocking the door at 9am. I asked him several times to ask her to stop calling so early but he'd just shrug and let her in. It was affecting both of us with the lack of sleep as I was having a fibro relapse at the time.
After a few weeks of this I was at the end of my tether. We both got up wearily and let her in and I told him to tell her. He refused so I asked her nicely if she'd mind not calling until the afternoon as she was getting us both out of bed. She went absolutely mental! Called me a selfish bitch, amongst many other things. And then stopped. For a few months.
I couldn't believe it one day when I heard the key in the lock and by the time I was downstairs she was going through my kitchen drawers!. I didn't quite catch her so couldn't accuse her directly but I knew.
My little not-leaving-key-in-door relapse was soon sorted.
Unfortunately, she was a major reason for our divorce. Still, she got her son back for a while. I often wonder if his new wife put up with her for as long as I did. I'm just happy to have got rid of her - though mutual acquaintances and friends tell me she still bad-mouths me whenever she can. She was doing it from the day we met so it's water off a duck's back now.
OP - it's obviously bothering you. Your husband doesn't look like he'll stand up to her and although some of the suggestions here are brilliant (but I'm bound now to get Amazon emails suggesting blow up dolls!), I don't think the ruder ones would be sensible with your DC in the house.
The Australia one is inspired, though.
It's the ideal time now, while you're doing the place up, to add a bedroom lock - or just upgrade them all to locked doors. Just hide the keys of all the others in the attic or somewhere. Or you could install a snazzy new fingerprint lock or keypad.
Her behaviour is NOT likely to change. You could try the alarm suggestion and it might shame her out of her nosiness, but I doubt it. You'd have to make sure it's set every time you leave your room - unless you have time to run up and set it when she arrives. See if you can Pavlov her into not going near your room!
The dropping in whenever thing needs to be sorted, too. It would drive me mad, but worse, it could put a wedge between you and your husband and if other tensions develop in your marriage it will make things a thousand times worse.