Nope. I'm in a very similar situation with my son's friend/our neighbor. His behavior is abhorrent. He's banned for the long haul. He is a danger to my upcoming newborn, and negatively influences my littler ones. Poor behavior has consequences, and if OP lets this kid back in, the bad behavior goes beyond electronics. Violence, spitting food and gross speech are not tolerated. I have a kid with special needs and he knows how to behave, he's been well taught according to his needs.
A mom's job is to protect her family and home. Not her sister's home, not her friend's, not anybody's. This kid is a tornado. Unfortunately, she's got a tough road ahead, but his visitation should be at a full stop. The little ones will indeed be impacted by his behavior.
That alone is enough. For my son's friend, we meet at the play ground, pool, birthday parties, the trampoline park- "safe places" that don't negatively impact my home or family's well being. Leniency is how this kid gets away with this stuff.