I think it is way harder to stay calm when someone is attacking my family than when they are attacking me --- must be that Mama Tiger thing, huh?
I suppose the logical thing is to wait until you are really, really icy calm and then talk to her about it --- let her know that you understand how stressful this all is for her and how hard, but that it isn't OK to take it out on you and your guy. What you guys do or don't do in NO way affects her --- it's not like you are "stealing" her weight loss ability or something.
On a more positive note, I think it is really cool that you and your beau are both losing weight together and it is TOTALLY cool that you are proud of him for what he is doing. Don't let this emotional crap with the wedding derail YOU from keeping up the good work, ok?