Others have said it already, but here's how I would put it.

She's going to drink heavily no matter what you do. If you tell her, she'll drink. If you let her come, she'll drink. Alcohol abusers will use any excuse--or none.

Anything other than absolute honesty is enabling her. Enforcing your boundaries is the only honorable way to behave in this situation.

I feel for you, though

Karen