It would have been best if she'd said something that would open the door for Silas to spit out the truth to Nina when they had some privacy, like Silas has been a very good friend to me, we spend a lot of time together etc. If it was a straightforward situation, which of course it's not, then there is simply no way for Sam to speak the truth at that time and place without being a terrible person, in my estimation even if was being shown as obvious that Nina is a Psycho Bitch From Hell, (beyond being portrayed by Michelle Stafford) Sam would still be on the wrong side of things if that was her reaction. Even if Nina were a sweet, frail paragon of virtues who is utterly clueless about Sam's attachment to Silas, she still needs to know sooner rather than later about his life in her absence.
Really though, showing up at a birthday party for the child of strangers to spring a back-from-the-dead surprise(!) pretty well screams nutbar
I laughed when Nina said that line about how Silas did not need to worry about rolling in over in the bed and crushing her, because I'm pretty certain Silas' wizened self worries that all the wimmins are going to do that to him. If she wasn't wheeling around, I think Nina would have just flipped bashful Silas over her shoulder and tossed him on the bed.