I would guess that you have an older sister or a younger one.
I bet you probably have a big sister or a little sister.
It seems to me like you might have an older or younger sister.
いろいろな言い方ができますが、次のように言うと良いでしょう。
ーI would guess that you have an older sister or a younger one.
「あなたにはお姉さんか妹がいそうですね」
I would guess that ... で始める言い方。
ーI bet you probably have a big sister or a little sister.
「あなたは多分お姉さんか妹がいるでしょ？」
I bet you probably ...を使った言い方。
ーIt seems to me like you might have an older or younger sister.
「あなたはお姉さんか妹がいそうだね」
It seems to me like you might ... を使っても表現できます。
ご参考まで！
カナダ 
