I agree with Rose.....

Your boyfriend was supportive when your family ignored you.....why do you need or even expect their blessings on your engagement when they didn't care enough about you to be supportive when you really needed it before.....what made you think they would change their ways now?

Leaving your mother would be the wisest thing you could do....she isn't capable of supporting you emotionally, why should you support her financially?

Sounds to me like your future with your fiance is much brighter for you than anything your family has ever offered you. Maybe your family will finally figure it out before your wedding but I wouldn't hold my breath...you need to focus on the happiness between the 2 of you & not bring into your life the depression your family is causing.