Is it permissible to lie to one's sister so she accepts help, knowing that if she knew the truth, she would refuse the help and consider it an impediment?

The basic principle regarding lying is that it is forbidden, and it is not permissible except out of necessity or for a need that is deemed valid by Sharia. Lying is not permissible merely to help someone in need. If a person eligible for Zakat refuses it, it is obligatory to inform them that the money being given to them is Zakat.