That was extremely rude, and I agree, that asking her did she mean to be so rude is probably appropriate.
If it continues, I'd suggest setting a boundary. "MIL, that was extremely hurtful. You will apologize for that remark and not repeat it or similar, or this visit will be over." Then follow through.
If she continues to be nasty, then you may need to stop visiting her altogether. I'd only recommend that, though, if she doesn't start behaving appropriately.
Infuriated by this man's attitude
So let’s have a thread for the summer lovers
I miss the woman my daughter was before she lost her husband

