I think depends.... If the kids are living full time with you, then u should step in and act as a parent. I would leave big choices for the hubby to tell the kids, but still be there and supportive. If the kids don't live with u but come every other weekend, then just police them while they are there with u, but leave big choices to the parents. I would act like the fun aunt. There to police accordingly, but to get to know them but nothing heavy.....
It can be done, you just have to unite with ur man and stand firm as a couple.