Gtb, you and your db can easily deflate her by letting her do what she likes with her will. Just say " well it's your money, why don't you go on a cruise or something?" If you two have a good chat about the will, and agree what you both think is fair, her will will have no effect - you can do what you want after she is gone. I don't mean to sound callous but there is nothing she can do.
Don't let her ruin your relationship.my dm also disliked any kind of relationship she couldnt control and it drove her crazy that I spoke on the phone to siblings and didnt report back.
If she thinks the will is her last control, she might decide to leave it all to your ds! Just to drive a wedge.
Don't stress, just talk to your db, enjoy your break and actively - this is important- dont allow her to be your topic of conversation with your db- continue to build your relationship with your db with his life, future plans, watch a comedy together you both like, your life, future, so it's not all about her.