Maybe she thought he wasn't ready, maybe she was trying to keep him from all the problems her husband had?? And mc was away and her out of town when we arrive back in town. Doesn't matter, they talked it out
When the MC first came back to town and reconnected with his family one of the first things he did was inform them of his plans at that board meeting. Even going so far as to take the time to sit down and convince his sisters to support him.
Hell, from what I recall I believe the mother even gives the MC some advice on how to convince his sisters. While knowing the entire time it didn't matter because she was going to vote against anyway.
So the mother knew exactly what he had planned. If she actually gave a shit about him, she would have told him before the meeting that he wouldn't have her support so he didn't look like a damn idiot in front of the entire board when she voted against him.
Instead she watched him plan, convince his sisters, go to the meeting and only then fuck him over in front of everyone there without warning him.