YOU SAID:

oh no my cousin washed my neighbor's clothes in my brother's flat

INTO JAPANESE

いとこが兄のアパートで隣人の服を洗った

BACK INTO ENGLISH

My cousin washed his neighbor's clothes in his brother's apartment

INTO JAPANESE

私のいとこは兄のアパートで隣人の服を洗った

BACK INTO ENGLISH

My cousin washed his neighbor's clothes in his brother's apartment