Mid-life crisis? Or just selfish/insecure?

I have decided to write about my issue in hopes of receiving some good advice or to just simply understand what it is that I am going through. I'm 23 years old and my parents have been divorced since I was 9, due to my mom cheating. My dad was a loyal, hard-working, dedicated father and husband, worked 2 jobs so that my mom could stay home and raise my sister and me. My mom re-married shortly after the divorce. He was an alcoholic, lost jobs constantly and frequently on unemployment, filing for bankruptcy a few times. They had a daughter together, my half sister. My mom stuck it out and felt she had to "change his life". When she realized it was not possible, I helped her go through her 2nd divorce. We got an apartment together and I helped her get her life on track, this was while I was pregnant with my first and only child. Although she was still talking to my sister's dad and trying to "save him" still, she started to resort to MySpace to get her self esteem increased and to meet new people. She began chatting with different men she would find by searching or by them contacting her. She began dating these men and bringing them into our apartment with my (at the time) 8 year old sister and newborn son, all while I was at work. She mostly dated men younger than her, by as much as 10 years younger. After dating a few of these guys and nothing really working out, she began talking to Brian, whom she is currently with now, who is 10 years younger than her. My mom is 46. They met on MySpace and went on a few dates. After 2 weeks of "knowing him" she had him move in with her and my 10 year old sister. Everything was fine in the beginning, he would cook for her, clean, do laundry, etc. because he was unemployed. She let him live with her for free. He treated her like a queen in the beginning and even won over our approval. Shortly after, he started becoming manipulative and controlling (which my mom denies). She was no longer allowed to have a MySpace account (she deleted it), he went through her phone and she was given the 3rd degree for going out with her daughters! Everywhere she went he thought she was cheating on him, even being at work. After arguments over the phone while she was at work, he would pack up his stuff and leave before she got home from work and disappear for days and then wiesel his way back in when my mom would apologize. To make a long story short, one of the weekends when they argued and he decided to leave, my mom made another MySpace account to find that he still had one. She contacted a few people that were his friends on there to ask them questions. This is when the lies were revealed. He lied to my mom about his last name. He lied about the number of kids he has, saying he has 2 kids when in reality he has 5 kids and owes about 80,000 dollars in child support. He said his mom died in November 2008 (at the time that they were together) and left for the weekend to go "clean out her house". His mom is still alive! He said that he was never married before and he is indeed married AND engaged to someone else! He told my mom he had an apartment of his own, in which he was really staying with some girl. So many lies were revealed! He had us all fooled. So my mom seeked my advice and comfort and I told her she needs to stay away. She listened for about a week. I called her a week after she found everything out and she didn't answer her phone from Friday to Sunday. Finally I got a text from her saying that they were back together and that she "talked to his family and he didn't lie about his feelings" for her. This is the reason why she took him back! It didn't matter all of the other things he lied about, just that he "didn't lie about his feelings"! Since then, I have told her how it was a slap in the face that she seeked my comfort and then turned around and let this loser back into her life and I have barely talked to her since. I'm worried about my little sister living there. She is now 11 and my mom is more wrapped up in herself and her boyfriend than she is in being a mom. My sister has gotten highlights in her hair, artificial nails, and has been wearing heavy eyeliner, eyeshadow and mascara. No parental supervision what so ever. Not only did he lie about all of the mentioned things but he does tattoos out my mom's home on strangers that he lets in, all while a child is there. I guess I'm writing this to try to understand WHY my mom would let this criminal/loser back in to her life and how I could try to make her see the light. Also, I worry about the well being of my sister. I think it has gotten well past a "mid-life crisis" thing. My mom and I had always had a good relationship until her 2nd divorce. I can't tell her anything or voice my concerns without her thinking that I'm trying to "bring her down". Why does she act like this? What can I do? Especially for the well-being of my sister? Help please!