Wa Alaykum Assalam and thank you for your question.

Initially, the best supplication (dua) is the one that comes from the heart with sincerity. Supplicate in your own language to reach a higher level of sincerity.

A dua does not have to be long but it has to be sincere and one of the ways to increase that is by waking up for tahajjud prayer in the last third of the night, let us say, about an hour before fajr. Pray two rakats and elongate your sujood while supplicating for your sister. Ask your family members to do the same.

May Allah grant her success in this world and in the Hereafter.

I will also make dua for her inshaAllah.