It sounds like its alot more entrenched than a simple mistake.
My children grew up with their elder siblings spending approximately 10% of their time in our house and 90% of their time at their mothers house, whereas my children have no choice but to spend 100% of their time at my house.
The step daughter hasn't been kicked out of home or rejected, but the op is saying the influence cant be around her son until there's reformation.
I've never rejected my step children but they know certain behaviors cannot be exercised at our house or around their younger siblings
I agree with those saying let her mother and father sort it out. There's no reason you can't be in your DH's ear telling him he needs to deal with it... but I don't really think there's any way you can involve yourself further and end up with a positive result.
As for booting her from your life... it's a 3DS. She wasn't injected heroin in front of your son or anything extreme like that, I don't think, "You're banned from our home because you stole a DS," is a fair punishment at all, and any father that would agree to that is a pathetic excuse for one IMO.
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