Hi Karen,
So glad this forum is helping you gain a more positive outlook. -- Bipolar is so difficult to get your head round at first -- for sufferers and friends/family. Fortunately there are some great info resources out there, which can really help you to establish more of a sense of what's going on, even when your sister doesn't want to talk about it and/or acknowledge it (and there may well be instances of one or both of these things happening on her journey to recovery). A great book I found was The Bipolar Disorder Survival Guide, by J. Miklowitz (I think those details are correct -- on loan to ex at present -- but you can find it in the resources/books section on the right-hand side of the page). That was a real education for me -- as has been coming here and asking questions or letting things out.
With the Lamictal, one great thing is that it has a detailed website -- http://www.lamictal.com/ -- with info on the med as well as bipolar itself. Nothing in the details says to avoid alcohol on this med, but it's possible your sister is on others with it, and there may be instructions for thos. The other thing is that anyone with bipolar should be careful about there alcohol intake. -- Alcohol affects moods and that can be a trigger for an episode. Also, some bipolar sufferers use alcohol so "self-medicate" because it dulls things out -- but of course they resurge and then some when you come down from the alcohol. Is there a particular reason you asked about this? Is this something that you worry about with your sister?
You have a wonderful day too. -- Your sister is lucky to have you there for her.
Rosie x