、。
My sister has two sons, so that means I have two nephews.
Literal
Younger-sister [topic-には] 2-people [possessive-の] sons [subject-が] exist so, me [topic-には] 2-people [possessive-の] nephews [subject-が] exist [result-ことになる].
~ことになる ('it comes to be that, it turns out that') presents the nephews as a logical consequence of the sister having sons — not a decision, but a factual outcome. The parallel structure (妹には…いるので、私には…いることになる) is clean and natural.