We are a family of four and I have one older sister.
I come from a family of four which includes my mom, my dad, my big sister and I.
we は普通に使いますよ!
ーWe are a family of four and I have one older sister.
「私は4人家族で、姉が一人います」
an older sister でも良いですが、この場合は one older sister の方が自然です。
ーI come from a family of four which includes my mom, my dad, my big sister and I.
「私は、母、父、姉、私の4人家族です」
I come from a family of ○. という言い方もできます。
big sister で「姉」
ご参考まで!