How to tell my mil to stop answering when someone talks to me!

you could tell your mil in casual conversation that people might consider you (not mil) rude because you are not answering questions directed at you. tell that its your mistake probably that you take a second or more to collect your thoughts, but to give you a moment to answer questions aimed at you directly.

don't make it a big deal. Later on some occasion if she forgets, remind her afterwards about what you had talked, but in a gentle way, not complaining but only as a reminder.

In spite of multiple reminders, if she still does it, there is nothing you can do without creating a scene.

 

It can get very annoying and sometimes you have to make the other person realize their mistake by not explicitly stating it. In your case i would suggest that you negate her a lot of times so she gets the drift. Example, if someone asks you about your DD and she replies "yes, because of blah blah reason.." say to the other person "well no, but it's because of this other blah blah reason."
Make up stories if you have to but she will understand and gradually stop. Good luck!