My mother can't stop worrying about who's going to take care of my brother who has a disability after she dies.
My mother is constantly thinking about what will happen to my brother who has a disability after she passes away.
この「母親」と「息子」が Jimiさんのお母さんとお兄さん（または弟さん）と仮定して言うなら次のように言うことができます。
1) My mother can't stop worrying about who's going to take care of my brother who has a disability after she dies.
can't stop worrying で「心配せずにはいられない」
「誰が兄/弟の面倒を見るのか」は who's going to take care of my brother
「母親の死後」は after my mother dies
2) My mother is constantly thinking about what will happen to my brother who has a disability after she passes away.
be constantly thinking about で「常に考えている」
what will happen to my brother で「兄/弟がどうなるのか」
「母親の死後」は after my mother passes away とも言えます。
ご参考になれば幸いです！
日本 
