Given you've clearly told her to change how often/what times she calls round and she's ignored you, I'd ask her why she does it.
Why do you come round X times a day and stay for X hours when I have asked you countless time to (insert your requests). Tell her you don't understand. Push her to explain it. Don't let her fob you off, keep asking for her to explain her reasoning. Do this every time she comes round until she explains/changes.
Either she'll get the hump and strop off - success
Or she'll tell you and there may be information that is new to you and something you and your husband can help with.
You aren't asking a lot. It's not like you're saying never come here again. You're just asking for it to not be all day every day. That is not unreasonable of you.
Also, when your husband is home, every time she comes round, go out. You've got an appointment. You know they've not had a lot of time together so you know they'd love to just spend this time together whenever he's home because you just know how hard it must be to not get to see each other and it can't feel nice for her to spend more time with you than with her own son so you'll clear out of the way and give them some quality time oh and with the kids too. Bye. See you tonight.
Let's see how great it is when it's just him, looking after the kids and entertaining his mum all day, every day he's home.
My guess is it won't take long for there to be changes and it's worth the short term disruption for you. 😁