Age difference in (step) children

Thoughts? 
My husband was a single father from 2005 until we married a year ago. We are interested in having more kiddos, but have an eye on the age difference. At this point, there would be a minimum 10yr difference. And because we're having a harder time conceiving than we expected, who knows where the timeline will end.
He's a great dad and I'm raising his daughter like my own. She's pretty accepting since she's never had a mom, and has been calling me Mommy since before we were married. But we're also concerned about the emotional adjustment a sibling will bring. 
Does anyone have any experience on this front? Insight and advice welcome, and good luck to us all!