It is your cousins choice to get married... have they forgotten it is her life? Your mom needs to remember she is not going out and "getting married" just to P people off. It is no ones choice but your cousins and to sit there and judge is horrible! why do people have to make things be about them, when they are in no way involved? Really, how is your cousin getting married interfearing with your mothers life? Tell your mom to quit being so negative and keep her mouth shut, It's not about her. If she chooses not to go to the wedding boo hoo. If I was your cousin and found out my Aunt was talking trash.... well let's not go there because what I would say would not be nice ;)
As for you I would simply tell your mom every time she calls and rants "Mom I love you, but my time is more valuble than to piss and moan about something that does not pertain to us. Either we change the subject or I'm going to have to go" or just the simple "Im too busy right now"
As far as supporting your cousin, OF COURSE!! It's not your life and to sit back and be negative is like your jelous of their lives. There is nothing more hurtful than family talking about you behind your back.
Why should your cousin have to defend herself, when it is a choice she is making about herself. You can't choose what age you fall in love, and when you find it you hold onto it. There are some people out there who have never truly fell madly in love, and settle instead. I am a lucky one to have the chance to experience full blown unconditional love, and this is from a man that use to be a boy at 18 and I was just a silly little sophmore at 15. Over 13 years together, 2 boys and still get butterflies in my stomach :)
*** oh and if it was me I wouldnt send out an email, that's how your going to get involved. When they call do something as stated above or just the "I don't have time for this"
Good Luck, your family sounds just as dysfunctional as mine! :)