This is legal terminology. The phrases "aiding and abetting" go together. It is a common phrase used in US law. The point is to include two possibilities. If the law only said it was illegal to "aid" someone, someone might defend themself by saying "Yes, I told them to do it, but I didn't actually help." The phrase "aid and abet" makes it clear that it is wrong to encourage someone to commit a crime, even if all you actually do is to talk. Other examples of standard legal phrases including pairs of words include "breaking and entering" and "cease and desist."

to aid = to help to abet = to encourage

You can “encourage” someone to do both good deeds and bad deeds, but you can only “abet” a crime. If you do that, you share in the blame.