Daval also stated in the beginning that this comic would be about "sharing" within the family, and that he wouldn't introduce NTR. Uncle Peter didn't fair too well because of his random insertion. So if most of the fan-base hated Peter's creation, and Daval3D has promised that he was going to eventually get "rid" of Peter, then why would he create a situation between Sarah, Peter, Silvia and some random "black doctor"? How do you think that would go over with the fan-base, who were already angry because of Peter? I think the conversation you highlighted above is a means to introduce Silvia and get us comfortable with the idea that she's slutty. Silvia's purpose is to introduce a slutty nurse who will be taking over Peter's care.
I don't think Sarah will even take the job now given the fact that her niece and nephew have absconded from her home. Not to mention, we didn't even see the end of Sarah and Silvia's conversation, assuming that there was one. Why? Because it's not important now. I think it is likely that Sarah will be looking for them, and perhaps catch Alice and David in the act. Remember, Sarah's sister entrusted her children to her. What sense would it make if Sarah just said, "oh, my niece and nephew have disappeared, let me just take a job with a woman who I suspected was trying to seduce my nephew, bring my uncle there, and have sex with him, and any random patient or doctor there." That really makes sense? Geez...