One twin girl has been having periods for over six month, while the other has never had one. should i be worried as their mother?

No: Even twins may develop at different times. It depends on body mass, exercise, diet and stress. If they are between 10-17 years old, it's normal both to start and not to have started. As long as your other daughter has started her growth spurt, is developing breasts and/or pubic hair, she's on track. If none of these are happening or she goes more than 2 years from these before bleeding, see an md.

No: No need to worry as every individual girl will start her periods on her own clock, though there will be a tendency for the starting age to run in families.