MY sister still chooses my ex over me?

My mother is bipolar and unmedicated. When I was 21 I had a child, and separated from his father when he was 6 mos old because he was not willing to leave the party scene and become a parent. While I was a single mom for a short time, my mother had a lot of control over me; I met my now husband and she lost that control- which she never got over. She did some horrendous things to try to split us up and gave up her right to be a part of our lives. She has this strange attachment to my oldest child, and when we would not let her see him (she was having him call her mommy, among other things) she became friends with my ex and bribed him with money and such in exchange for bringing my child to her during his visitation. My sister also chose to turn her back on me and became “best friends” with my ex. Now you’re probably thinking, what did you do/ there must be a reason why your family turned their backs on you. I would think that too. But the answer is nothing, except marry a great guy. 7 years later, my husband and I have a 5 year old as well as the 8 year old, and we are very happy. I just became an RN and my husband is working on his BA. My husband family is very supportive. Over the years, my ex has established himself in my place with my family. He has holidays with them, has lived with my sister several times, and even had our court battle funded by my mother (which they lost). Recently, my sister has gotten 6 felony charges for drug dealing and is out on bail, waiting for sentencing. After several years of estrangement, she approached me, expressing her love and asking me to put my home up for her bail. I refused but told her I would like to have a relationship with her and her daughter. She disclosed that my ex is on methadone, has stolen/betrayed her repeatedly, and has warrants out for his arrest. She also told me she was finished with him. I have been spending what little time she has left trying to rebuild our relationship; but came to the undisputable conclusion that everything she has told me is a lie. When confronted, she showed no remorse, apology, or embarrassment. She was pretty much like, “oh well, you got me.” Now she is moving back in with my ex, who she now says she loves and is her family (though she denies any sexual relationship), and she expects me to just accept this. She even asked me to help her pack- while he was there! I finally told her enough was enough; she obviously has no regard for my feelings and absolutely no respect for me. I am willing to support her while she is in prison, and give her daughter (6 years old) love and stability to help her through this tragedy that is her life (both of her parents will be in jail); but she must stop her relationship with my ex and stop lying to me. She has not responded. My problem is that I would like a relationship with her and my niece; I would like to help them get out of this situation and into a better life. Is it even worth it? I know a grown person cannot change their character- especially if they do not want to- but is it foolish to say I can accept her as she is, when her world is so different from mine? What does it mean when a sibling has no feelings for another? I am afraid to invest myself emotionally in my young niece, and she in me, to have her taken from me at the whim of her mother. My sister wants me around for some reason- she says she wants a relationship- but her actions scream a completely different story. This is forever long and I apologize, there was just so much needed to explain to paint a thorough picture. Any sincere comments would be welcome