Is it OK that my husband still considers his ex-wife's niece to be his own?

These people live hundreds of miles away from us in Conneticut, he never hears from them on a regualr basis, I've never even heard about this "niece" of his who is about 6 or 7yrs old, until recently. Here's the deal. My husband's son by his no good ex, has to go to CT for the summer. It is not something that I want, but with us having 5 kids and twins on the way, having 1 go away for the summer is going to be a welcome break. However, my husband's ex wife just asked him today if his "niece" could come down HERE for the summer, while his son, my stepson, is up there. Now, this makes absolutely no sense to me! I can't even believe that he would actually have me consider this! I don't know the littel girl, neither do my kids, and my husband is at work for 11 hours every day. His ex claims that the girls parents are going through a rough time. Whatever. Anyway, am I being unreasonable, or is he just asking for way too much from me?