Is it permissible not to inform one's parents that the fiancée's father drinks alcohol, in order to avoid their rejection of the proposal?

Informing your parents about the condition of the girl's parents and brother is not obligatory. If the girl is religious and of good character, and you are capable of distancing her from a corrupt environment, then your marriage to her is more deserving, as it involves marrying a religious woman and helping her to remain steadfast. This is based on the Prophet’s (peace be upon him) saying: "A woman is married for four things: for her wealth, for her lineage, for her beauty, and for her religion. So, succeed with the religious woman, and may your hands be dusted."