just personaly, I could never date someone who I my husband could see as being part of our family. I am new to this and I know I've only had the one girlfriend, but if my husband did not want her as part of our family, long term, I wouln't be dating her. That is just how I feel, but I agree with anyone it is more about what you and everyone involved want. If you are open and honest and talk about what you all want you will figure out what the relationship "titles/roles" are suppose to be. Lastly I wanted to add that if the third party involved isn't willing to even talk (e-mail, phone, anything) than the honest/openness will never come and I don't think the whole thing will work at all.
Hope you works it out.