Molly was protecting their privacy. Had she mentioned she was a surrogate for her sister, there is no doubt in my mind Molly and TJ would be furious.
They seem to expect Kristine to sit in a corner in the dark until the baby is born so no one knows she's pregnant. Kristine should be concerned that any affection or time she'll want with her niece/nephew would responded to with "your not their mother."
I've seen photos, and it looks like the actor who plays TJ can be fun, but his scenes show him as grimly professional, disapproving, angry or all of the above.
There is literally nothing Kristine can do that doesn't make TJ and Molly angry.
Karen L.