My younger sister cannot live without alcohol. What should I do?

I'm new here... Don't know how this really works but I truly need help.

I have a 24 year old sister, very smart, beautiful, people envy her achievements, etc.
I knew she liked to party and stuff. She's always been a fun girl, very sociable.

Last year, we had the idea to go on a family vacation, my family is my mom (who is a widow), my sister and myself.
We went to this very exotic and beautiful place in the jungle, absolutely breathtaking.

Because we were not really on work vacation, we stayed only two days.
It was cool until we got there.

We arrived at night of a Friday i think, so I suggested to try out some of the exotic cocktails from that region (I love herbs, plants, anything new and 'different'), my mom, my sister and I ordered one drink each. I was chatting, laughing, my mom very chill, having fun, my sister was on her phone the entire time, not even had interaction with us and ordered a second drink saying things like 'Damn, this is LIVIIIIIIING'.

We were leaving and she wanted to have another drink, we took it as like 'Oh, she's excited to be here' and nothing else.

Next day we went to the hotel in the jungle with a view of this awesome lagoon.
My mom and I were chilling and admiring the lagoon and all the green surroundings, my sister stayed with us for 6 o 7 minutes with us (out of the 35 minutes we were there just admiring nature), she asked me to take pictures of her with the landscape and said 'I have to go charge my cellphone'.

She disappeared for like 30 minutes... I returned to the room to go to the bathroom and then I went pass the cafeteria and I see my sister sitting ALONE with the most miserable look on her face with two big cocktails...

I ignored it and returned to that place with the great view to relax.

Later that day we went swimming to some place in the lagoon. It was amazing!! My mom was relaxing, I went and swam, play, etc. I told my sister to get in the water, that it was so nice and fun. She looked at me and said something like 'I'm not sure'. I was OK with it, after like 30 min I got out to lay in the sun for a little bit and I saw her with this HUGE bottle of beer, again, alone, sad face, just looking at her cellphone.

I started to get weirded out...

And for the last activity we went to this town, the arts and crafts town of the place we were visiting. It was cloudy so there was not much to do.

My sister looked bored TO DEATH. The tourist guide told us that there was this very cool museum we should visit, I encouraged my mom and sister to go. My sister was like 'Yeah, ok, let's go'. We get to the museum and next to the entry was this cafe/bar and she bought herself a cocktail, we had to go into the museum so we rushed her, she drank it really fast and we started exploring the museum.
Leaving the museum she told us 'What about another drink? Maaan, i need it, hahaha'. My mom started crying and I having a nervous breakdown, i had to give her some of my anxiety medication because I really didn't know what to do, she started biting things and crying while screaming.


I would understand if you are at a party and have several drinks, man, I've done that when I was like 17-22... but, in a family vacation, with no music, no friends, just your mom and your sister, while being at a heavenly place.

I confronted her, I was so angry that she made my mom have this breakdown, I told 'YOU ARE AN ALCOHOLIC', she was unfazed and told me to stop saying idiotic stuff.

Later, when my anxiety medication calmed my mom, she confronted my sister but VERY KINDLY and in a loving manner. She responded OK but she finalized with '...I get what you say, I understand where are you coming from and I'm sorry if you feel bad but I KNOW I'm just normal, I'm OK, I DON'T have a problem, this is normal, I'm not gonna stop drinking'.

What do you think?
I've also seen her IG stories and she drinks almost daily... I even saw a story where she is drinking rum on a flight :/
Is she sad? addicted? I don't know and I am so worried, every sunday morning she is passed out in her bed smelling like alcohol.

BTW, I'm not like 'ewww, alcoholic!', i LOVE my sister, i would do anything for her, I just feel that something is going on with her, she seems so unhappy and I feel like acohol is not going to make her happy.