I don't know the dynamic of the relationship with your sister, but there are several possible options. If you are comfortable being around alcohol without being tempted to drink, you could do nothing other than have your non-alcohol beverage of choice for yourself and if asked you can say you don't feel like drinking tonight / are taking a break / are trying some healthy lifestyle changes / etc.
If the established pattern is getting together for the specific intention of drinking, is there a way you could contact your sister before hand and give the same message by just working it into the conversation?
I'm kind of a fan of the diplomatic strategies, especially in cases where alcoholism hasn't caused a lot of outward, visible damage to work, relationships, etc.
I use this strategy a lot. My decision to quit drinking luckily coincided with the start of Lent. I started a week before Lent and initially joked about doing a trial run to see if I could do it for 40 days. As Lent has progressed I've been able to make it a light hearted conversation about how it's going with coworkers, family, etc. Now the conversation is turning to how it's really turning out to be quite nice and that I'm thinking this might be something I'd like to stick with on a long-term basis. I can sense that people are slowly catching the drift about what's going on, and I've been pleasantly surprised at how genuinely supportive they have become. I'm even to the point now where I mention participating in SR for support and have even dropped the "a" word as appropriate with certain close and trusted friends outside of family.