This shouldn't be a huge issue either way. The gate attendants may or may not let your DS board with the lower numbers. Several posters have reported being allowed to do that in similar circumstances, but it really is up to the discretion of the gate attendant and there have been plenty of reports of passengers not being allowed to do the same thing. There's no harm in asking, but don't throw a fit or panic if they don't allow it; just go to one of the alternate plans below.
As PPs have noted, you would be able to have your DS and DH swap boarding cards (wait to do it until the gate as you can't do it before security, however) as the gate attendants do not check IDs. I suppose if you tried to board with a pass that had a male name on it someone might notice, but otherwise the chances are slim and none that it would be an issue. If you ask about doing that, however, they probably will say you can't do it.
The absolute worst case scenario is that all of you--or your DS and one of the other two--board at B12. Since your other numbers are at the very end of the A group, there are only the family boarders and B1-B11 between the two. There's almost no chance that you will not be able to find three seats together--maybe a bit farther back in the plane than you want--boarding at B12.