Why have children?

Hello, this is something that's been concerning my sister and I for quite awhile. I have 2 older sisters, in there early 30's. One who is single and the other, who this post is about. She has 3 kids and has no interest in them whatsoever. She has an 11 year old who my parents and my other sister raised since she was 9, because she worked full time. They just moved out 2 years ago. She has a 2 1/2 and 6 month old who she didn't raise at all. My sister called her mother in law over to raise them. Mind you, she's a full time stay at home mom. She has no part time job, nor school. She convinced my mom to move into her apartment complex and literally throws her kids on her, every single day. My mom watches them, complains to us and even keeps them overnight. My sister agrees that she needs to take responsibility, it's not like she's working or anything. She was never the type to like kids. It's sad because she has no relationship with my niece. She says that her mother doesn't do anything and makes her watch the house, cook, clean and watch her brothers 24/7. We are seriously concerned. Yesterday my mom told us she couldn't get up, her kids were crying all night. Mind you, my mom is in no state to even do anything. She's extremely ill. So my mom was like, "I went over, cooked and cleaned and came back. She's not well." Last night I went over. She looked at me while she handed her 6 month old to my brother and said, "here hold him. I have to throw up." I was shocked. My niece was like, "yup there's a 4th one coming. Perfect." It's just ridiculous how she does not care for them, when she has the time to! So my question to you guys is, why have children?