brother and sister
Je ne pense pas que Tom et Mary soient frère et sœur.
I don't think Tom and Mary are brother and sister.
I don't have any siblings.
Toi et moi ne sommes pas frère et sœur.
You and I are not brother and sister.
They are brother and sister.
Elle et moi sommes frère et sœur.
She and I are brother and sister.