Translation:I have seven younger sisters.
Not really. They switch between pronunciations depending on the counter, and you kinda just have to remember it. なな is used for normal counting, age (~歳), and minutes (~分), among others. しち gets used for things like months (~月) and hours (~時).
Happens a lot with 四 (よ/よん or し) too.
İt doesn't really make sense in English to just say "there are three younger sisters". Younger than who? The sisters are the subject of the sentence, but there is also an implied older sibling. Without any additional context we should assume that the implied topic of the sentence (the older sibling) is yourself. This is true for many of the sentences you will be translating, as it is common in Japanese to leave the topic unspecified when it can be implied.
In certain contexts "there are" would make sense: "I have a bunch of first cousins living nearby. Among them, there are 7 younger sisters... so I've pretty much learned every song in Frozen by now." Although you aren't necessarily referring to your own little sisters(s), you're still talking about your own family, so you still avoid honorifics with "sisters".
Got it in one. You use いもうと to refer strictly to your own little sister when talking to other people. When talking about other people's younger sisters, you use いもうとさん. If you wanted to just say that the seven women are sisters in the same way you say "those people are siblings", you'd use しまい/姉妹.
You'd be using 姉.
Generally speaking, when talking about your own family members to other people you use the 'humble' forms (母、父、姉、兄、妹、弟)
When talking about other people's family members you use the polite forms (お母さん、お父さん、お姉さん、お兄さん、妹さん、弟さん)
When addressing your own family members, you use the polite forms if they are older than you; although sometimes people use the informal, shortened versions (かあさん、とうさん、etc) When addressing family members younger than you, you generally use their first names.
If you're asking for a term to refer to all siblings collectively, it's 兄弟（きょうだい）. 姉妹（しまい）if you only have sisters. If you just want a term to refer to specifically older siblings of both genders (or only to younger siblings)... I'm sure there is a term but I dunno what it is, sorry ^_^"