Being taken advantage of

Whit
So my mom has been watching this lil girl since she was 3 months old (she'll be 2 in October) and naturally we have gotten attached to her. She calls my mom mama, we buy her clothes, keep her overnight, she's even spent Thanksgiving and Christmas with us. Her mother is a year older than me and also has a 5 year old and I completely understand being a single mother is hard, but I feel that this girl is taking advantage of my mom and my family. She works as a prison guard and there are times where she has to work overtime (I get that) but she'll text my mom and say I'm on my way to get my child at a certain time and then doesn't show up until 2 or 3 hours later. This past weekend she went to Vegas and came to pick up her daughter at almost 2 in the morning. The next day she took her daughter over to my nieces house around 5:45 pm because she wanted to take a nap and her daughter wouldn't let her. She didn't come back to pick her daughter up until 3:45 am. My niece is only 15! One time we kept her daughter for 2 weeks straight. She never once called, text, or even came by to see her her offered to bring us any clothes or money for watching her daughter. One weekend she said that she had to work a double and asked if we could keep her daughter overnight and of course we agreed. Well my sister, niece and I were just scrolling thru Facebook and snapchat and come to find out she wasn't even at work she was at a party. It's a very sad situation and it's very frustrating. I happen to love this lil girl very much and I honestly would adopt her if I could. I'm not sure if I should say anything to her mother or not. I understand everyone struggles and needs help, but I just feel like she is taking advantage of my family. I don't want to say anything because I don't want her to get mad and then stop letting us see the little girl anymore and just let anyone watch her. 
- Sorry so long. Just my rant for the day.