。
I have one older brother and two younger sisters.
Literal
I [to-は] older-brother [subject-が] 1 [counter-人] [and-と] younger-sister [subject-が] 2 [counter-人] exist.
Listing family members with the existence verb いる (for people). The と here is the conjunctive 'and' joining the two clauses. Note 兄さん vs 兄 — the さん-suffix here is unusual for humble reference to one's own brother, but it's casual speech and still works. 妹 ('younger sister') without the さん is standard.