I don't see
Tamsin as masculine at all, but
Artemis is to me, somewhat. Especially because most obvious nn's are
Art and Arty (blech).
Tamsin has a very UK/British vibe to me while
Artemis is more exotic and mythological. A better sibset, imo, would be
Tanith &
Artemis. I don't see
Tamsin &
Tanith as similar in style, but I guess because they look and sound a bit similar, I associate one with the other. So
Tanith was the first name that popped into my head. :b