It's odd, but Hawkeye has always been the motherly one. She believes in tough love, but it's love all the same.
Every one of the men under Colonel Mustang's immediate command has at some time or another made the comparison in their own mind, and reached the realization - the reason that Mustang commands such loyalty is that he treats them like family, genuinely like family, not like people wish families would treat them. It means that he will scold them and send them to bed without dinner when necessary, that he will tease them for their annoying habits and be horrifyingly sarcastic at them on a regular basis, but also that, when it comes down to it, he will do anything to protect them.
Hawkeye will back him up on it.
The analogy is perhaps best of all for Edward and Alphonse; they are, after all, the youngest. With everyone else she's a mother by nature; those two, she took on deliberately. After the death of Nina - an incident that haunted her for weeks, but that haunted Edward for years - she had taken them home, with the excuse that they could hardly go back to Tucker's house, could they? and she didn't live on the base, so she had a bigger house than anyone but Hughes, didn't she? and Hughes had a new daughter to look after and didn't need the trouble of houseguests. So they came home with her, just for a little bit. She made Edward a cup of cocoa and found some extra blankets and saw to it he did get to bed. He slept without dreams. She'd drugged the cocoa. Tough love.
Somehow they never got around to moving out again. When she moved to East City, following Mustang, she bought a bigger house there than one person really needed, and although the brothers were assigned a room in the barracks, they used it rarely.
Hawkeye is motherly, even though she's anything but feminine. Her co-workers treate her like one of the boys, and invite her out for drinks, but she always stays sober enough to beat them at darts, and walk them safely home. Colonel Mustang doesn't drink, but Hawkeye will take him out anyway, sometimes. Occasionally she's taken him home afterwards. They make love like an old married couple, and there's never been a pretence of romance. Romance, for Mustang, is an act put on for the benifit of important men's cute secretaries. Romance, for Hawkeye, is something she's been too old for since she was twelve and outgrew her pink party dress.
They'll never speak the words, of course, but they do love each other. They're family.
The brothers became part of the family very quickly. As much as Mustang layered on the sarcasm when he was talking to Edward, his loyalty had extended to the boy almost at once. Hawkeye had known when he came back to Central City, three days later than her from a side-trip to Riesenburg, and told her what he had seen there. Mustang makes decisions quickly and well. It is part of what makes him such a good officer. He had made a decision then, even if he had not known it himself. Alphonse was quiet, helpful, and occasionally enigmatic; he was also instantly likeable. Edward, however, kept himself to himself, and burned with the fire of an ambition tinged with mania; he drew people to him, like cult leaders drew people to themselves, with the light of something not quite earthly. It would have taken a sharp eye indeed to see the child inside him.
Hawkeye has very sharp eyes. So does Mustang. And family is important.
Alphonse didn't seem suprised when she brought Black Hayate home, although everyone else had seemed quite suprised when she offered. Alphonse was almost as good at understanding people as she was. She hoped he was good at understanding canines as well. At least there were plenty of people to help her take care of the puppy; even if he couldn't live in the office, there was no reason he couldn't stay there during the day sometimes. Perhaps once she had him housebroken. She was quite looking forward to the challenge of raising a puppy. Besides, once Edward had gotten over his shock he'd had the sweetest smile she could remeber from him on his face, and Hawkeye knew the value of a genuine smile. Edward had very few of them, and tehy had always been for his brother alone. Perhaps if he began to think of Black Hayate as the family dog, he'd begin to realize that Alphonse wasn't the only one who wanted to see him smile.
Hawkeye had realized early that she wasn't cut out for marriage, or having children. She's grateful to have realized this, and found a better path for her ambitions. Still, the paths of her life are the paths of a wife and mother; her ambitions are tied to Mustang, and her days are taken up with his work. Their family surrounds them, and it's a better family than most women dream of.
She makes a wonderful mother, after all.