I would tell her, "MIL, you come around far too much and I feel suffocated. You need to only come around when we have arranged it." Then give her a time e.g. 2pm on Sunday (ideally when DH is around).
If she turns up outside that, say, "MIL, what did we discuss yesterday/last week/two minutes ago? I can't let you in now. I'll see you on Sunday."
Then next time, "is there a reason why you're ignoring what I'm asking you to do?" Let her hopefully squirm.
Next step would be, "you are seriously interfering with my right to privacy and to enjoy my home. If if you carry on harassing me I will have no choice but to take further advice." Do not answer the door again ever.
I would also say to DH, "I have told you your mother is here 5-6 times a day banging on the door, disturbing me and the children. It is way too much. I am happy to see her once/twice a week/month (whatever you think) but I can't have this current situation continuing. You have so far refused to help. Why is that?" Let him hopefully squirm.
Then, "I think it would be better coming from you but if you don't tell her to back off, I will. Which would you prefer?"