co-parenting issues

I have two stepsons, each has a different mom as my husband was married twice before me.  I really just need to vent because it's frustrating.  The last year has been especially tough. The younger son his mom just up and left and moved to a different state, she also has not paid child support from day one so she owes a lot of money.  She's chosen to not be that involved, maybe calls once or twice a week.  The older son, he's 14, his mom lives about 3 hours away and in the last year she has cancelled so many weekends, at first it bothered our oldest a lot.  Now it's like a joke to him and he just shrugs it off when I have to tell him she cancelled again.  She use to be really good about taking him, but within the last year she hasn't.  She's late on child support, not sure by how many months but has paid before.