I believe it's not always due to a hormonal abnormality. Of course she should be checked by an equine reproduction vet, but you might not find anything. If there are no other symptoms, it could be simply this mare's behavior.
Most likely things will quiet down if you are not boarding another horse. Some people give mares a medication called Regumate to stop a lot of undesirable behavior. I always hope I can manage a situation by making some small changes, rather than using a medication for the rest of the animal's life.
If you choose to buy your own gelding later, it's possible things will settle down given time. However, it also sounds like you have all the animals close together and this often causes problems.
Having a separate run in and door for the husband's horse will prevent what you're describing; dividing your pasture in two will keep the horses from being so close and getting on the mare's nerves...especially if her door and his door are well separated.
Mares very often can't be pastured or penned with geldings. There are too many fights and problems. At one barn, all the geldings fought over the one mare there - when she was gone at 4h, peace reigned. The minute she came back, it was kick wounds, injuries due to falls, and plenty of bite and cut wounds.
Many mares have a lot of protective or mother behavior toward other animals. It isn't unusual. My mare has a fit every time the calves next door are moved to another pasture.
All mares can't always be turned out with geldings without getting a lot of upsets. Some can, some can't. The secret with horses is finding out what works for the animals you have and sticking with it. 'What works' isn't always exactly what we want, but it's almost always possible to find 'what works'. Vets and professional trainers can often offer suggestions.
In the old days, it was common to hear that the best thing for an overly motherly mare, was to breed her and let her have a foal. I'm not sure it always or usually worked, but like many 'rural legends', it was repeated often enough and believed too!. But with all the excess horses in the country today, we can't afford that kind of solution.
