You don't have to have your info searchable. You can make it so that ONLY those who you Friend can see your info, your pictures, your stuff. I have it so that people can see my basic info and my user picture, and people can see what I post on the pages of others (can't control that), but other than that, ya gotta by my friend to see more.
I don't think FB works that way, to create an account for two people alone.
OP, what if you replied with a big cheery "hi! so happy to see you here!" type of post in the thread that sis and SIL were talking on? Assuming you *want* to do that.