Specifically, look up Rule 1.7 regarding conflicts of interest related to current clients ... keeping in mind that for Jeff Sessions, his "current client" is the United States of America, not Donald Trump.
This is why the stories about Trump wanting Sessions not to recuse himself were probably true ... because Trump probably has no idea about how these ethics rules apply to a situation that he is always seeing (understandably) in political terms.
I would submit that this is a grey area at this level and one that has been consistently ignored by Democratic administrations. And if they want to play with knives in the gutter - that is where we need to play also.