Sim 3 - new baby boy

At this time , the son from the family kind of have a deep relationship with another sims who is living in their house. The son and cheria have sucessfully deliver a new baby boy into the family which i call babel . It seem that we can choose the baby trait if we send the mother to attend for seek medical advise for 200 dollar.

And it seem i have a hard time finding how to get both the son and cheria married although both living in the same house. Any idea how to do that?