He might be maturing early but that does not usually mean your females are also. Breeding young can negatively impact the females health and overbreeding, especially when young, can even kill a female. Best wait until she is over 18 months of age before you attempt breeding the lady's and no more than one breeding per 3 months and no more than 3-4 breedings a year.
For the girls health, you may want to look into having 2 tanks - a bachelor pad and a bachelorette pad. Considering they are siblings, no breedings should be done.