As you must know, most Shelters sterilize young puppies before they are adopted. We adopted a small breed pup from the Shelter, who I am pretty sure, was about 4 months old. She is fine. Had she not been spay, I would have had it done before her first heat cycle. Many don't have a heat cycle before 6 months but they can have it sooner or later then that.
Some studies show there is less of a risk of mammary and uterine cancers in females spay at a young age. In your case. She could be done now or wait until she is 5 months and hope she dies not come in season sooner. Have you consulted your Vet?
If your male dog is not neutered, I would strongly suggest you consider that also. Will deplete some of the hormones and there would be less risk fir cancer also.