I don't disagree that Barb has given Jenelle too many chances and enables/is somewhat codependent with her. However, I was under the impression, correct me if I'm wrong, that Jenelle had Jace spending weekenda with her because it was legally mandated. Would Jenelle have gone her merry way and left Jace alone, if Barb had put her foot down? Maybe-for awhile. Jenelle wants what she wants when she wants it, just because she is petty, spiteful, and possessive.
I think it is really easy to judge Barb and say she should've/shouldn't have done this, that and the other, without being in her shoes. At the end of the day, Jenelle is still her kid, and I would guess, not having been in her situation, it is tough to just wash your hands of your child, fuck up though they are.