Hi Spade
Nice domain.
BigSis.com
Positive
- old domain according to who.is since 2002
- easy to remember, pronounce and spell
- brandable
- world wide known words
- catchy sound, the words flow fine together
- short
Negative
- cant find any really

- domains sounds feminin, so it might "scare" of the male (sorry, just a thought)
I agree with Diabro that it would be a nice advice site/community targetted girls. But I also think it could be used as a site selling/marketing surveillance products, like a counterpart to "Big Brother"?
I think at least mid-high $x,xxx, but if you could find an end-user ex. an organisation or government supported program with big bugs, who has teens/girls as there targetgroup, this could be 10 times as much, imo.
Good luck and best regards,