Personally, I would suggest talking to your mother in law and clarify your position. Tell her how you feel about this. She is also a married woman and I can bet she has come across such ladies more than you in her life. Let her know you could put the random aunty in her place but you don’t want to and instead want to clarify that you and your husband have a certain plan.
Now IF, the lady is still acting naive, tell her straight up. Lady, I don’t want to spend the rest of my life with three kids in a little room that is owned by my in-laws. I would really appreciate if you stopped thinking about me and focused on how to make your own life better.