
There are no guarantees re having the younger one spayed and what's going on. However, I'd get the younger one spayed in any case - there's enough to have persuaded me that the more seasons a bitch is allowed to have the greater the risk, and it is only a risk, of mammary, ovarian or uterine cancer later on and the ever present risk of a pyometra. Most of our bitches were not spayed until retired, and we did have just the one who developed mammary cancer. Not something I'd wish on my worst enemy.
I'd do what you can to avoid the growling to escalate to a fight - once bitches fight, there is often no way back. Males fight to mate, bitches fight to survive. Could be the ears causing all this however. Dogs know when something is wrong with another one and her ear infection could have made the younger one feel the need to defend herself, before anything happened?
We have had generations of related hounds under the one roof, mostly bitches, mothers, daughters and grand-daughters etc. We only had one challenge situation between males - one young lad coming up to 2 started at his older uncle. Try as I might, I couldn't get him to settle down so had to home him which I found better than having to make sure they were separated when we were out.