Can I do anything legally to my 36 year old niece who bought alcohol for my underage daughter and her friend?

Yes. You can have her arrested for furnishing alcohol to minors.

Sure there is something you can do - you can have your niece arrested for assault and battery and for providing alcohol to a minor. Bear in mind that this is likely to cause a pretty big rift in your family. Your sibling isn't going to be happy that you had his/her daughter arrested and your daughter is going to be angry with you for interfering with her relationship with her cousin. If I were in your shoes, I would call a family meeting to discuss all of the issues going on here. I would instruct your niece that she has to be responsible for paying all of your daughter's medical expenses from the trip to the ER. I would inform your daughter that she is now grounded with a curfew and not permitted to hang out with her cousin unsupervised for 6 months. I would advise your niece to attend a couple of Alcoholics Anonymous meetings so she can learn what excessive drinking can do to a person's life. She also needs to learn what drinking at a young age can do to the still-developing brain of a teenager. I would say all of this in the family meeting and tell your daughter and your niece that if they do not comply with what you have requested that you will press charges against your niece for assault AND against your daughter for underage drinking. My guess would be that neither your young daughter nor your old-enough-to-know-better niece want to have to face a court of law so they will toe the line with your demands. Be prepared to stand firm - for the sake of both your relatives.

I answered your first question but there were less details yes you can get her arrested and charged and what not however your daughter might get charged for drinking underage so consult a lawyer about that first.

I'm not a lawyer, but I think you should be able to BOTH file charges with the police for a criminal violation and also sue her in a civil court.

Call the police. Providing alcohol to a minor is illegal.

You can take legal action but why sue your own family? There has to be another way. Maybe make her pay the medical bill instead of taking it to court.

You can file a complaint, but that is about all. You are not a party to this as your daughter is a legal adult. She can file a complaint as well, which will have more relevance since she is a party to this event.