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No offense, but...well, judging by the many suggestions similar to the above, it's clear that many of you have never lived with crazy people.
Here's the thing:  YOU CAN'T REASON WITH CRAZY.
The OP's mom won't tolerate any such statement without a fight because it takes her out of control...
This statement is the truth! She has a gambling problem because she refuses to admit she does not have CONTROL over the situation. I can't NOT use the word gamble or say, maybe you shouldn't be out doing the spinning slots... she automatically goes nuts yelling at me and the "MIND YOUR BUSINESS" thing comes in. She spends when she's depressed, but she believes being depressed is a made up thing, and she's just sick. 
I have to agree, though some things are illness related, ...but really, the woman is just plain CRAZY.
OK, I agree you can't reason with crazy; so 
don't try to reason.  Simply say something to the effect to "I'm not going to talk about that" and change the subject.  And do it over and over and over, as necessary.  DON'T talk about gambling; period.  Or do an intervention.  But nothing in between.  
Mom is, at least to an extent, correct, it IS only her own business, and as long as she is not hurting others, she is allowed to act stupidly.  Obviously though, she IS hurting others; the OP as her daughter and whipping post is hurting; the other daugter and husband may be hurting; the grandchildren may be hurting.  Finances and transportation are obviously issues.  If Vfem wishes to cut her mother out of her life, fine.  That is up to her.  But I did not get the impression that that is what she really wants.  If she did, she probably would have already done that.
My intent was to suggest some coping strategies to help Vfem stay in control of herself and her interactions with her mother.