calling all custodial stepmoms, stepchildren, and bio dads. someone help me out here!

Okay so I am a custodial stepmother, meaning i am the primary caregiver for my stepsons. My husband and I have full custody of all four of our children. The oldest is almost twelve and his mother has been gone since he was 18 months old. she has visitation once a month for two hours but it has to be supervised by either my husband or I. she has never shown up for any visits. She randomly shows up maybe once every two or three years. Last year my son started to get curious about her and was wondering why she never comes to visit or calls him or anything. So I drove him two and a half hours away to meet her and his brothers. we were only there for maybe two or three hours. She is ordered to pay child support 158 a month. she NEVER has. she somehow always gets out of it. she has been arrested twice for it. my SS doesn't know we don't explain that to him he's not mature enough to understand all that (maybe he is and I'm just being overprotective) but anyway she finally got a job and her child support is getting taken out of her checks. She contacted my husband and I last night and asked us if she could pay for me to legally adopt him so she wouldn't have to pay child support and plus she's about to move across the country and she would rather take care of all of this and get past this part of her life before she leaves. What do we do? We have discussed as stepparent adoption before but the way that she worded it and all that it just sickens me. what the heck. someone give me some advice. btw my SS recognizes me as mom calls me mom and her her name but he still loves her and refers to her as his mom he just doesn't see her often enough to call her mom he says it makes him feel weird because she doesn't do what mom's do.