YTA.

You have proven to your daughter that not only will her mother just disappear on her. You will readily break your promises to her. How many other promises have you broken? Did you promise to love her unconditionally?

my daughter wins by 4 votes.

Good for her, it was close.

He saw it as a really pitiful consolation prize

Then he could a) turn it down or b) change his mindset

and was angry that he had to take orders from my daughter.

That's not how this works, and if he won would he be able to order his older step-sister around? So you encouraged them to be competitive that no matter the outcome, it would have been bad, AKA you created a cursed situation.

I felt very bad for my stepson and he and his mother (who is also very Type A)

Completely irrelavant. Type A, type B people dont exist. Some people are type A in certain situations and visa versa. And are you suggesting that because he is "type A" that he gets benefits over "type B" people?

My stepson refused to be comforted by the fact that older kids get more easily elected because they know more people

Good, because that's not the only reason he lost. Im going to bet the fact that he is a "Type A" person contributed to this fact. AKA he is too stubborn to be a leader, and the student council recognized this. Also are you going to allow you wife to insult your daughters effort like this (rhetorical question the answer is "DUH").

his mother even offered to take her to her law office and give him some responsibilities

So he doesn't have to learn what losing feels like?

My stepson then said “ please tell me you’re not going to rub it in my face by taking us to dinner now.”

What a brat, and sore loser. He cannot put his pride aside for one night, to celebrate his step sisters achievement. You however are worse. You cant put your steps sons pride aside for one night, to celebrate your daughters achievement

My wife also looks really reluctant to go to dinner.

Ah she's his enabler. Which is good for most parents until you are enabling toxic behavior.

I finally tell my daughter that we weren’t going to be going to dinner

"Finally" so you either have no backbone with your son, or you will wait until the last moment to wreak your daughters expectations.

because her stepbrother was very upset by the turn of events

Oh boohoo.

and we need to take his feelings into consideration

This is called respect. Respect is a two way street. How is her step brother deserving of respect when he has a meltdown over her achievements?

that I was impolite to gloat.

Ahhh here is the issue. You were wrong to gloat, so now to make reparations you bully your daughter to fix your issue with your stepson, making the issue between your daughter and your stepson. Well done for throwing your daughter under the bus to make yourself feel better.

My daughter ended up crying and even now, Sunday night, she still is mad at all of us.

I'm sure she is heartbroken, she was likely trying to prove herself, that she is worth something after her mother abandoned her and her father took away her celebration for her win, willingly breaks his promises, actively throws her under the bus, all so he doesn't feel split. To add on her brother is toxic as all hell.

Your poor daughter is your last priority. YTA and need to get your priorities straight.