We just used the normal wedding wording..
So and So requests the honor of your presence at the marriage of....
to ...
the son of ....
date
location....
Our wording was really tricky b/c of both of our mothers have passed away and we wanted to honor them as well as not offend our step parents. Do a search on the internet for invitation wording, or pick up the Emily Post wedding etiquette edition. That is a great resource.