You need to open discuss the fears that are leading her to assume thay she will loose you to him. Be prepared, she'll have enough am by now. It's sounds like your sighs and tones have mostlikely given enough away, and don't expect her to be ignorant. She feels the need to play ignorant to it so she doesn't only create drama outside of herself. And it's working, she's making herself the mater so she can play ignorant when she isn't.
You might be telling her a tousand times that you love her and that everyone DOES want her to be there, but She shouldn't be so dramatic everytime.

It's conflicting messages. You need to sit down with her and have a mother daughter heart to heart. You need to tell her that you realize she is only acting this way out of an intense fear to loose you, and knowing it'd the worst thing in the world to happen for her, you will never put her through that, but if she needs to learn to love your happiness. To look past her own fears and seeing that you are happy, even if it's not in the way she wants it to happen. And if anything where to ever happen to your marriage for what ever reason, she has the responsibility to take you back without a "I told you so". Tell her you love her and you want her to be your mother, and that's part of the deal. ...which means you can't just let her die alone, either a Good nursing home or a home close by.

If it's not worth that then just let her go. Just know the bond between your mother and you ist stronger than any Husband. You need to imagine the love you'd feel for your own child. Most mothers will go through that bonding and to desert them, is to desert our fellow women, even the crazy ones love. The reason she is acting like that is BECAUSE she loves you so much, it's juts being filtered through crazy: Change the filter Girl! *