YOU SAID:

My sister has a child and that child’s aunt is my uncle’s sister

INTO JAPANESE

私の妹には子供がいて、その子供の叔母は私の叔父の妹です

BACK INTO ENGLISH

My sister has a child, and the child's aunt is my uncle's sister.

INTO JAPANESE

私の妹には子供がいて、その子のおばは私のおじの妹です。

BACK INTO ENGLISH

My sister has a child whose aunt is my uncle's sister.

INTO JAPANESE

私の妹には叔父の妹が叔母の子供がいます。

BACK INTO ENGLISH

My sister has an uncle's sister and an aunt's child.

INTO JAPANESE

私の妹には叔父の妹と叔母の子供がいます。

BACK INTO ENGLISH

My sister has an uncle's sister and an aunt's child.