due to lack of time ,sometimes we help our younger sister in her project or decoration work which she has to submit in school for marks. many times the work is done more better that she can do.and she gets marks for it .is this halal ? what if the time too less, can we help in that case?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

If it is a habit that such a girl is helped in such activities and the school does not object to this, then it is permissible, Allaah willing, to help her according to the extent that is permitted. However, if it is commonly required from the girl to depend on herself only in doing the activity, then in this case it is not permissible to help her at all. 

Allaah Knows best.