You need more than one line, I think, but you don't need an epistle either.

Have them write about what they know of your relationship with each other, how they are in support of it emotionally, as well as being aware of your plans and being prepared to support you two financially if necessary. They can even be specific, stating they are willing to provide housing, if that is what they are offering, or monetary support, if that is the case.
Remember, be specific rather than use generalities. What they are telling CIC shouldn't sound like what they would offer to any stranger in need, but what they are willing to do for their child and the person she loves.
Hope that makes sense.