Translation:We are sisters.
~たち (達) is a suffix used for pluralizing people, such as turning "me" わたし (私) into "we" (私たち) or "friend" とも (友) into "friends" 友だち (友達). However, しまい (姉妹) is the specific word for "sisters", made of the kanji for "older sister" + "younger sister". The same goes for "brothers", which is きょうだい (兄弟); "older+younger bro", though this one is used for "siblings" in general as well (as most patriarchal languages do).
Of course, you can use 'boku' and 'boku tati' with your friends !
私（わたし）（わたくし）（all people okay）polite, be able to official situation
僕（ぼく） ?（male）polite than おれ
俺（おれ） （male）many male use. but I not recommend at the meeting in the company (firm, business), official.
There are many words, etc.