You might just need to be firm with your cousin, that you will not ever do the school work for anyone in a situation like this.

And that the stress and now the pressure from her and the girl is not good for your health at all.

If the girl can't cut it she needs to help herself - a job, free English lessons etc
She really should go to the school and ask for help there.
as others suggested.

I'm miffed that your cousin is putting you in this spot.