Translation:My father does not have a younger sister.
Also for future references if he does not have sisters period, you could use jie mei ( a combo of younger sister and older sister) which logicaly would translate as neither younger nor older sister. How practical! Wo de baba meiyou jie mei. I thouhght I would share that tidbit. Xiexie
When there is no measure word, it's ambiguous as to whether it's supposed to be younger sister or younger sisters. Therefore, both can be accepted. Although, in this case, since it's stating that the father does NOT have younger sister(s), it'd be odd to include a numerical value to indicate how many sister(s) the father doesn't have.