Two Plus Two Newer Archives - Need Help - Family Situation with Younger Sister

To preface, my sister is 17 and I'm 21. She recently moved out into her own apartment with a friend. Last year one of her friends, who was 12-13yo at the time got pregnant, and the father was a semi-acquaintance of both mine and my sisters. He was 18 and his name is Dominic. This guy has broken into dozens of cars, fights people for little to no reason other than to prove he's a badass, and at the time had drank too much of the 'I am a thug' kool-aid.

So about 12 months ago, I get a call from my sister while I was at a poker game. "Nick, the lights are on... I think someone is in the house." I basically told her to move her car out of the driveway, go down the road and turn off the car and try to disguise the fact that someone just got home. Then call the cops, and I'm on the way.

The cops get there the same time I do, and they go in. They find Dominic downstairs in my parents room completely barred out (Xanax), with the back door wide open (suspected that his friends escaped). His neck was draped with several thousand dollars worth of my mom's jewelry. His pockets lined with everything he could stuff in there, including my graduation ring and the expensive present that I had just purchased for my mom. In his hand was my dads handgun, that was loaded which he immediately dropped when the police stormed the house.

I'm not sure of the charge he faced, but it was at least a 15+year jail-term because of the firearm.

Fast forward to about 2 weeks ago, and I hear from my mom that all charges against him were dropped. I knew why. There's only one reason why. My sister had been trying to get my mom to drop the charges for months, but she had been successful, and I was shocked.

My sister is extremely intelligent but she is brilliant at a mixture of arguing and manipulating. You can go into a room with her pissed as all hell, and she'll find a way to turn the tables or calm you down. I really can't put into words how good she is at this, but getting my parents to drop charges should be an indicator.

My three older sisters were just as outraged as I was, and I had a serious argument with my mom and she admitted that she regretted her decision. Another problem arose.

I found out Dominic was going over to my sister's apartment which my mom is paying the rent on. I basically accepted the fact that the charges were dropped, but I strongly believe that the guy who broke into my parents house with a gun and could've possibly killed someone should NOT be at my little sister's house. Period.

My mom is gullible and easy to manipulate, she's a great person but she gives people too much credit. My dad is out of the picture since they are separating after 23 years of marriage, so me and my older sister are taking the stand and saying no it's not going to happen.

Paige, my little sister uses, "Dominic is a changed man, he's got a kid, he's off of drugs, he's being paid $35/hr at the plants." as her argument.

I'm going to assume that most people feel the same way I do about this, but I need help forming an argument to counter this.

The decision on whether or not my sister should be around him is mine, as my mom has said that she'll do whatever I feel is best in this situation. I love my little sister incredibly, and I don't want to be on bad terms and I want her to understand why we feel this way other than, "it's best."

I've got little to no ammunition because this guy dropped off the radar as far as my friends are concerned. I haven't heard how he's behaving since the incident. Also keep in mind that he's got a kid with one of my sister's friends. She says she feels that she is keeping him out of trouble, and she is there for him to have someone to talk to about his 'family' problems.

So how would you approach this situation? How do you counter her argument about him being changed? Do you even agree with my position?

The recent phone conversation ended with me telling my sister that if he wants me to trust him then he's got to talk to me, instead of her arguing on his behalf.

...but seriously, FFS, the guy broke into my parent's house and could've killed somebody is how I feel, but she won't accept it. She just retorts with, "people change, you can't constantly hold grudges and you should learn to forgive because we all make mistakes."

Edit: I already told my mom that if he goes over there she should pull rent payment, and she agreed. She basically swore to me that she'd do it if I felt it was necessary, and he's going over there. Again, I could really piss off my sister if I wanted to but I don't want to go down that road...

The friendship between me and my sister means more to me than this douchebag.