I would share BB with my sister as it is a good forum for me and may be for her. I would not tell her my username, but she may well guess who I am and that wouldn't bother me unless she brought it up IRL in front of others.

I would not share BB with my sister-in-law because all she would post would be "moo". I'm sure you understand what I am saying. (I also wouldn't tell her my username in case she came on and saw this post.)