my parents are wasting all my money on my good-for-nothing sis!

My 30 y.o. sister still lives with parents! She had a stable job only once, briefly after college. She’s a meth addict, so no one wants her for long. They had an agreement with the father that she’d be paying them rent because she doesn’t know when she can move out. I think she upheld the agreement for maybe 3-4 months while she had that job. Ever since then, I doubt she has contributed more than $10 to the monthly family budget. My parents are retired and aren’t rich, while I’ve made a career in banking, and at 29 I support my own family of four and also give my parents $600-700 a month. I never ask questions, I know that my money is spent on my parents AND my hideous sister… Last year we opened a joint account for my mom and dad, and I put there $4000 as a gift for their wedding anniversary and my father’s 65th birthday. I hoped they’d go somewhere, they weren’t able to travel ever since my sister returned from college and wreaked havoc in the neighborhood. I thought that in any case they’d be wise enough to spend the money on themselves: house repairs, health, anything. Now I accidentally find out my sister had access to their debit card and was using it freely. I knew something was amiss when I visited them last month! She was wearing new clothes and looked smug. I was stupid enough to think she finally found a job or perhaps met some sugar daddy to support her addiction. Then my father takes me aside and asks if he can borrow a certain sum of money. Turns out they’re so short on cash they can’t even afford a proper Christmas celebration this year. I was furious! When I knew they’d gotten only 1/5 of my last year’s gift for themselves and the rest trickled down to her “to pay for her debts”, I lost my temper. I told them I wouldn’t send them any more monthly checks while that monster lived in their house. I told her to get out. I know it’s not technically my house to say that, but what the heck! I’m not working my ass off to give my money to a thirty year-old addict! My mother burst into tears, and I left. It was two weeks ago. My wife fully supports me. But now, in hindsight, I feel awful. I have to be bad to my family right before the holidays… My sister texted me, calling me names. The ludicrous thing is that my mother probably thinks I’m wrong and ungrateful, while her poor sick daughter is nearby and is, of course, a darling. How’s that fair?