I've maintained for years that she is, but can understand why people don't think she is. I'm here to present my side of it once and for all, just so I can feel productive today within my dying fandom.

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The top woman's name is Big Mama Gerogiana from a new anime called Gangsta. She is the owner and operator of a brothel called Pussy (wow original I know) and works right alongside her girls. She wears a short red silk boustier type dress, has a white fur ruff around her shoulders, long black silk gloves and smokes. Also wears a slicked back bun, no bangs or forelocks or any of that nonsense. She's sassy, blunt, no time for horseshit or games, and also peddles information on the side while taking care of those who take care of her. Also has a mole, near her right eye, in addition to gold stud earrings and a necklace. We get to see her interact with her clients (not sexually) and also her girls.
Then there's "Blue Sophie" from an anime called Monster. This one's a bit tricky because she's not actually the madame but she is a working prostitute in Czechloslovakia. However- She is overweight, wears a very low cut dress, lots of makeup and jewelry, same slicked back hair, smokes, and also is brash and a no-nonsense personality. She deals more in information now, as old and fat as she is, but she does still accept johns though we don't actually see this in the series. We don't see her interract with anyone other than a handful of people, none of which are other whores or the madame.
Then there's our beloved Chris Mustang, sole owner of the Christmas Inn. She has several girls under her, and we get to see her interact with them as well as Roy, and off camera with Grumman. She smokes, supposedly drinks, slicked back ponytail, huge ostentatious jewelry, low cut dress and fur ruff collar on her big overcoat. We know she deals in information and women, but this is where the fandom divides.
Some say she has a hostess club. And I can understand why- in Gangsta, we see Big Mama coming down a staircase at Pussy, and all the girls there are wearing next to nothing. One woman even has star shaped pasties covering her nipples, low cut bikini style panties and heels on. We don't see that at the Christmas Inn. However, the general feel of FMA:B is that of the 30's-40's. Vanessa is dressed like Kelly Bundy from the late 90's, in what at even that time was referred to as slutty clothes. Madeline, the one Roy drives around town with, is even shown wearing a super short dress with a plunging neckline in addition to the slicked back ponytail and the fur ruff on the collar of her white coat. The Christmas Inn may not have been as blatant as a club called Pussy, but it is definitely a brothel, and Madame Christmas is most definitely the madame. Anime in general always portray some characters very stereotypically, as archtypes, and Chris Mustang fits this archtype.

(click for bigger resolution)
The top woman's name is Big Mama Gerogiana from a new anime called Gangsta. She is the owner and operator of a brothel called Pussy (wow original I know) and works right alongside her girls. She wears a short red silk boustier type dress, has a white fur ruff around her shoulders, long black silk gloves and smokes. Also wears a slicked back bun, no bangs or forelocks or any of that nonsense. She's sassy, blunt, no time for horseshit or games, and also peddles information on the side while taking care of those who take care of her. Also has a mole, near her right eye, in addition to gold stud earrings and a necklace. We get to see her interact with her clients (not sexually) and also her girls.
Then there's "Blue Sophie" from an anime called Monster. This one's a bit tricky because she's not actually the madame but she is a working prostitute in Czechloslovakia. However- She is overweight, wears a very low cut dress, lots of makeup and jewelry, same slicked back hair, smokes, and also is brash and a no-nonsense personality. She deals more in information now, as old and fat as she is, but she does still accept johns though we don't actually see this in the series. We don't see her interract with anyone other than a handful of people, none of which are other whores or the madame.
Then there's our beloved Chris Mustang, sole owner of the Christmas Inn. She has several girls under her, and we get to see her interact with them as well as Roy, and off camera with Grumman. She smokes, supposedly drinks, slicked back ponytail, huge ostentatious jewelry, low cut dress and fur ruff collar on her big overcoat. We know she deals in information and women, but this is where the fandom divides.
Some say she has a hostess club. And I can understand why- in Gangsta, we see Big Mama coming down a staircase at Pussy, and all the girls there are wearing next to nothing. One woman even has star shaped pasties covering her nipples, low cut bikini style panties and heels on. We don't see that at the Christmas Inn. However, the general feel of FMA:B is that of the 30's-40's. Vanessa is dressed like Kelly Bundy from the late 90's, in what at even that time was referred to as slutty clothes. Madeline, the one Roy drives around town with, is even shown wearing a super short dress with a plunging neckline in addition to the slicked back ponytail and the fur ruff on the collar of her white coat. The Christmas Inn may not have been as blatant as a club called Pussy, but it is definitely a brothel, and Madame Christmas is most definitely the madame. Anime in general always portray some characters very stereotypically, as archtypes, and Chris Mustang fits this archtype.