I pray to Allah to guide your brother and your sister-in-law and bless your nieces and nephew to be saved from this hellish life.
There is no need to place the blame on black magic or influence of jinn or anyone else for the situation you have described. It is all due to drinking and turning away from the remembrance of Allah. Allah said that drinking is a vice, and those who wish to prosper must shun it. Thus, we read, "
And furthermore, you already said they do not pray or establish the remembrance of Allah; and Allah says, "O YOU who have attained to faith! Intoxicants, and games of chance, and idolatrous practices, and the divining of the future are but a loathsome evil of Satan's doing:' shun it, then, so that you might attain to a happy state!" (Qur'an: 5:90).
Furthermore, as we are told, time and again, in the Qur'an, there cannot be any true peace or salvation except through remembrance of Allah. It is clear that your brother and his wife has turned away from it; so, what else could they expect, when Allah says, "But as for him who shall turn away from remembering Me-his shall be a hellish life and on the Day of Resurrection We shall raise him up blind." (Qur'an: 20:124)
Therefore, as adults, if they wish to gain peace of mind and improve their situation, they need to be willing to change, and in order for them to do that they need to be self-motivated. So, the best advice you can give them is to convince them to go for counseling.
As for the children, they need to be saved from this highly destructive atmosphere, which, I am sure, is taking a toll on their emotional and spiritual health. So, you need to do everything you can to get them the necessary help. You should do that, without further delay.
You may also read the following answer on renewing faith.