T.T.
I have always hated the word stepmom. It tends to imply less love to me and I don't think that's true for most.
Here's your roll...coming from both sides of the fence: you are support to your husband, but not the decision maker for your step son. You cannot believe everything you hear because I can tell you from experience it's only one side of a three sided story. (Believe it or not the xMIL and your husband CAN embellish to suit...just sayin)
Is your s-son happy? Is he taken care of? Is he fed? does the mom do drugs? Is he always dirty when he comes to your house? Because unless he is then you guys don't have a leg to stand on to get 'custody" unless she agrees to it.
Not paying attention to a child isn't grounds for proving that the child is neglected. My daughter said the same thing to her s-mother and they took custody of her only to find that she was a nice manipulator and now they are seeing what I was having to deal with for the last 4 years. It's not easy.
Its hard to stand back and watch what's going on with the other parents, but you have to choose the love the kid more than you hate what's going on. And then do the best you can with that.
I am sending good thoughts your way.