"I have seven younger sisters."
七人 しちにん shichi-nin.
This page has a fun video to help memorization of people counters http://genkienglish.net/genkijapan/japanesecountersforpeople.htm
The subject isn't implied here. 妹『が』七人います the が particle marks the subject. Note that this sentence would literally translate as "There are 7 little sisters", but with a implied topic 私は this becomes "There are 7 little sisters of mine", or you could use "exist" which in some context in English is a synonym of "be", then "7 little sisters of mine exist." Anyway, this unusual but close translations might visualize you which the subject of this sentence is.
Japanese often does not speak in full sentences but statements and quotations. A conversation which a first statement is removed is spoken as such: A: 今日は試験だ = Today has the exam B: ジョンは？= What about john? (Literally read as "john have?", impossible to translate) A: ジョンは明日 = John has his tomorrow (Literally read: John is tomorrow)