Hi sfs, she's drinking, you know that, so you don't need to prove it to yourself or her. For a real alcoholic sticking to 3 drinks a week is just too hard. You can do it because you're not an A, but I couldn't do it because I am (sober).
If I started having 3 drinks a week every receptor in my brain would light up like a Christmas tree, and it would only be a matter of time until I was back to full-on drinking. Abstinence is the only way, given her history, but if she's bargaining for her 3 drinks a week, she's not anywhere near to understanding her condition or wanting to become sober.
You're getting between her and the drink, as so she's becoming nasty and defensive, which makes you feel worse, and isn't good for anyone. Try to step back a little, and decide how your want your life to be. No matter how much it's harming her, she's chosen to keep drinking. Where does that leave you?
PS: Involving her mother sounds like you're telling on her. Keep it between you 2 if you can.