I think it is because deep down she realized that she really isn't all that. I mean think about it, after how many years and she still couldn't get anyone else with high status to court her. Even the Duke whom she gave loving eyes to didn't even want her. She always believed that she should live the high life but in reality, she can't so she's pissed that her sister will get to live the life she believed she deserves. Bua is the type of girl that do not know the definition of enough. Do you think she would really be satisfied marrying Haym? She'll always want more, more money, more ranks, more of everything. She wouldn't stop with just her husband too, it'll go all the way down to her great grand kids.