Sister has changed.. (long)

Ok so this is kind of a vent.

I have a younger sister who is 25 and she's a mom of a 4 year old boy and a 5 year old girl and she used to be a great mom who did stuff all the time with my niece and nephew but now not so much. So this is where the story takes a turn for the bad. She works at family dollar and had to move to a different store and while there she met this guy and they started talking and finally went out. Shortly after she started dating him her attitude started to become really bitchy and bossy and it got so bad I had to move in with my mom (I lived with my sister for 2 years and helped her with bills) and then she started to play games on her phone all the time (like weird animated fighting games) which my sister never was into games, weather it was on the phone or whatever. We met the guy and no one in our family likes or trust him (we think he's a big fat liar). He's supposedly a college graduate who has a degree, but here's where it gets unbelievable he's 37 years old, lives with his parents, and works at arbys and he hasn't had his own place since getting a divorce from his ex wife that he doesn't even pay anything too except like $100 a month in child support (he has 2 boys). Now over the last year of them dating my sister has become obsessed with video games (she bought a xbox one that was meant as a Christmas present for my niece and nephew but she has it in her room and she plays in constantly and won't even let them touch it), when she isn't working she is playing on that stupid Xbox or hanging with him. When he is around she pretty much stays in her room with him all day and lets the kids run around outside by themselves. She doesn't even take them to the park anymore or take them out to anything. She will get home from work and go straight to the game and play it til 3-4 am and won't even get my niece and nephew off to school in the morning because she's to tired. She only wakes up in the mornings if my mom calls her and she won't even get up she will just tell the kids to get dressed and after they leave go back to bed til 12pm and my family is getting very frustrated with her because she's putting this stupid guy over her kids. They just recently got engaged and we are hoping she does not marry him because I have seen him grab my niece and nephew by the arm and pull them down to the ground while she just watched (that made me furious). We tried talking to her but she just gets an attitude and won't listen. I'm seriously getting very mad at her and it makes me sad when my niece tells me she wishes her mama would play with her and do stuff with her like before. Don't know what to do anymore but if anyone has gone thru this with a family member, advice would be awesome! Sorry it's so long.