So when things are working well, what are you doing (or not doing)? I am wondering if in all the hustle of a busy life, his new behaviors are getting ignored. I do NOT suggest heavy praising, but maybe some time taken to walk him through some type of 'this event happened, you did <nice thing>, your sisters and parents reacted <this way>. It felt <this way> to you. conversation. I find walking kids through connecting the dots with cause and events is helpful.
You can also think about and watch for what his "payoff" is when he's like this. Does he like the reactions? (So then can you teach your daughters to ignore it or downplay it so it will stop.) Does he like the burst of attention on him? (Can you find other things to burst attention to him? ) Is he upset and is this his way to let people know? (Lead him through a better way to express what he's upset about, what he wants to happen and how he should say it.)
Also, something that he's done for years - don't expect it to be fixed once and then it's perfectly okay from then on. So if he does it for a day or two - strengthen the habit. If he falls off the horse, so to speak, help him get back on. Learning new actions is the same process as learning any new habit.
Two good books, if you like reading are "Siblings Without Rivalries" and also "Mom! Jason's Breathing on Me." Jane Nelsen is also a good author, but I don't remember offhand if she has a book that is specific to siblings.
Hope things get better!