Rosa and Susan gave you wonderful advice, and I agree, you have to forgive and move on. She acted completely inappropriately, but she did apologize, which is important. If her parents are very upset with her, you can leave it to them to decide if they need to talk about it with her more.
The primary thing of importance here is that you found your birth mom and together, you and she and the family made a special occasion of it. The fact that a soon-to-be family member infringed on it and made a fool of herself hurts her more than anyone. Many, many families have someone who drinks too much at family get-togethrs. It's just a fact of life. But some of them do grow up, or get help if needed.
Celebrate the year with your mom. What you can't get back is all those years without her. But the occasion of the anniversary is still incredibly special. It's your future sister-in-law who was demeaned here, not you or your birth mom or your very special anniversary. And I'm sure everyone who was there feels the same, Bexiboo.