2 week old and staying drunk??

So, idk what to do about this. I know its not really my place but, my husband and I are really concerned about her. We're really close. You ladies may have seen my post about my 19 year old sister's husband leaving her at 8 months pregnant. Well, she had the baby and he's about 2 weeks old now. He hasn't contacted my sister at all about the baby since he was born and just made a Facebook post about taking the woman's kids he left her for to waterfalls and the park. My sister has a new boyfriend now too. He's been a good dad to my nephew for the most part.

Anyway, point of the post, she has been leaving my nephew with my mom and step mom every day since they came home from the hospital. He's only stayed 2 nights at home with her. Last night being one of those nights. I have a couple of friends that go to our local bar almost every night. Not just to drink but they have corn hole tournaments. The girl has texted me inviting me to come down and asks me a lot where my nephew is because my sister is there drinking.. this has been almost every night for 2 weeks. She also texted me 2 times she wasn't at the bar asking me to come hang out and bring beer with us. I'm really worried about her and my nephew. He's tongue tied so she has to work with feeding him. He was 5lbs 7oz at birth. Obviously he's gonna lose weight in the first couple days. But he's been losing weight more than normal. He's 2 weeks old and gained 6oz in the 3 days in a row my mom had him and lost 4oz at his dr appt after that while my sister had him.

Do I have a justifiable reason to be worried? Or am I over reacting. She never used to drink until the baby came home..