selling job on the patriarch [closed]

This is a very slangy usage, imho merging two somewhat different usages.

To do a job on someone / something usually means to deliberately do something that has a harmful impact on them.

But to do a selling job [to, with, or on someone] means to persuade someone to accept some proposition (often using unconventional or extreme methods) that they would normally resist.

In the cited context, the group knew that the patriarch wouldn't be naturally inclined to accept their proposal (perhaps he was initially even hostile to it). Hence it would be difficult to "sell" their ideas to him (where to sell means to convince others of the truth or value of something - compare I don't buy that! = I don't believe that!).

Note that it's not necessarily implied that the group should offer inducements (a good post, a bribe, or whatever) to the patriarch in order to persuade him to support their proposal. They might achieve their aim by convincing him this would be the morally correct thing to do, for example. Or by threatening dire consequences (rebellion?) if he refused to support them