Well, it's not really confirmed, but, eh, yeah, I guess Pharmercy's probably going down. It's not like anyone actually expected it to happen though so, oh well, what can we do. Fanart will always exist even if canon acceptance doesn't.
I don't think we should get overly-specific with tags. They're legally sisters but not blood related so just tag it. Saves the trouble of having to explain to someone who doesn't know much about the series that they call themselves sisters, are recognized publicly as sisters, but are not actual sisters so when they do the nasty it's not as bad as it seems.