he said they are the type that don’t listen no matter how many times you tell them something (I already know this) so they will keep asking to look after DD
This is the key to your problem here. Reasonable people change their behaviour after being told their behaviour is a problem for you. They understand that if they don't, you'll withdraw from the relationship, and they don't want that, so they accommodate your preferences.
Unreasonable people do not respond to being told and have to be shown through action, eg withdrawal from the relationship.
Once you have told someone that their behaviour is a problem for you more than a couple of times and nothing changes, you know you're dealing with someone unreasonable. Now the onus is on you to show them that you're serious about your boundaries.
In regard to asking about having DD overnight, you haven't given them a clear boundary yet. Maybe a good one would be: "I know you're keen to have her to stay but we're not comfortable with that until she is weaned overnight, which we're not expecting to do until she's 1 (or whenever you genuinely think you'll be prepared to consider it). When we're ready for overnights, we'll come to you to arrange one, but we don't like being pressured, so please stop asking."
If they can't accept that, draw a harder boundary. "Kathy, I've told you when we'll be ready to consider it, and asked you to stop pressuring me. Continuing to pressure me makes me feel as though you don't respect my wishes, and makes me less comfortable with the thought of overnight stays. If you keep asking, the date at which we're prepared to consider an overnight will keep shifting back."
If she asks again. "We've discussed this, the earliest date at which I'm prepared to consider an overnight is now 13 months." If she asks again: "I told you the date would continue to move back the more you pressure me. Now it's 14 months." At some point your MIL is going to realise that the only way to get what she wants is to respect what you ALSO want.