Is a piece of string long? It's totally dependent on how the kids have been brought up and how involved she is with them and how much she wants you to see them. my ex has two kids and I loved them almost as much as my own, she was the only thing that caused stress in our relationship not the kids.
That was my experience though, you just never know. It's like asking the question, "I'm going on a date with a woman, will I marry her?" Who knows. Life is a risk, you either take the risk and live a little or don't and protect yourself. Neither decision is wrong, they're just decisions. Good Luck!