If it is an e-ticket. It is strict "No". It will be considered as Ticket less travel and will end your realtive in trouble with fines.
If its is PRS ticket(ticket taken from counter), then before entering the train, talk with TTE. 99% he will allow you. Advice from my side is talk before entering train. If you don't sometimes TTE may be in bad mood and may say no, fine etc and all. So better talk before entering train.