David and Aaron are married, wanted a baby. Aaron had some kind of potential hereditary ailment(?) he could pass on, so they decided David would be the dad. They looked into surrogacy etc and it was going to be expensive and lengthy. After a glass of wine, on a whim, Nicolette offered to be their surrogate. And I think they agreed on all co-parenting.
Nicolette is Jane’s daughter. Aaron is Chloe’s brother, Aaron also had brothers Mark and Tyler in the show a while ago. Tyler is/was in a relationship with Terase and Brad Willis’ youngest daughter Piper and Mark is married to Brad Willis and Lauren Carpenter’s (Lou’s daughter) daughter Paige.
David and Leo are Paul’s twins he didn’t know about. Apparently he slept with Kim Taylor in ‘86 which seems far fetched, as she was friendly with Scott Robinson not Paul.
All off the top of my head, so hope it’s close enough and that it helps slightly.