Ooops I forgot to mention that the dream specifically stated that he's not marring his half sister to the son of his vassal for political or financial benefit, but he will use the political and financial benefits that he gains from the marriage, rather he's doing it to ensure that even once she becomes a adult she's have a very good life
In case your wondering why The Senior Baron would arrange a marriage that would guarantee that his half sister has a very good life rather then using the power, wealth, influence and reputation he has to guarantee she has a very good life I have to admit that the dream didn't say why he didn't use the power, wealth, influence and reputation he has to guarantee she has a very good life