My reading was that it was Maeiv who doesn't vote. The other three are voting on her proposals.
Viewing post in The Survival of Sarah Rose comments
That wouldn't explain why whatever you support gets 0 votes when you get no one's support. It should still be getting Sarah's vote, so there should be 1 vote for it. Also, I did a lot of testing and can confirm that Maeiv does vote.
If you choose to keep the armies separate (something none of the others initially support) and get no support, it gets 0 votes. Get only her support, it gets 1. Get her support and one other, it gets 2. Get everyone's support, it gets 3. This is true for the others as well, so everyone definitely votes the way they say they will.
I have to assume it's a simple coding error/oversight. My guess is when you talk to each person and get them on your side (or they're already on your side), a counter is increased by one. When you get to the vote, the game just checks that counter and uses that number as the vote count, when it should be adding one to that number to account for Sarah's vote.