It's time for some tough love. I worked with a child that argued quite a bit and the family was a bit worse off than yours. I decided that it was so important for this child not to rule the house, that I had to be drastic.
I put him in time out every single time he did something that wasn't ok (arguing, pushing his sister, talking back, say "aaaauuunnn" in my face disrespectfully...anything that he did that made me cringe inside, he had a time out for). At first I thought it was too harsh, but he alway got a warning (unless it was hitting, and the warning was for the first time I heard the behavior and he got time out, after that, he got no more warnings and went straight to time out)....Example:
He was frustrated about something, turned to me and said, "aaaauuunnnn!"
I said, "That tone is disrespectful. If you do it again, you'll get a time out. If you want help, ask me for help (I always replaced the negative behavior with an appropriate one so that he would know what was acceptable).
"aaaaauuuunnnn!" Time out.
The first three days were REALLY hard. I made a rule that he couldn't talk in time out and every word he said gave him one more minute. He ended up sitting in time out for 22 minutes! (He kepts saying, "aaauuunnn" and I kept moving the timer up. He finally gave up and sat quietly). I also set a timer that would ding. That ding was like a party for him. He would get up and have a giant smile on his face. I would ask him to explain why he was in timeout, make him apologize to whomever he wronged, give them a hug, both would say I love you and they would resume playing.
After about a week, he ended up going from time out 6-10 times a day, to 2-4 and after two weeks, maybe only once or twice. He started using his words and being polite.
His mom thanked me immensely. He actually started asking them to use their manners. He was much happier and they all were able to enjoy each other more.
The difficult part was when his mom got home. She wouldn't discipline the same way I did (excuses: She just got home and he misses her, whenever he'd make his disrespectful sound, she wouldn't put him in timeout in his room because she didn't want his room to be a hateful place [it wasn't, he understood timeout was just a time...not a place]) She made so many excuse that he would look me right in the face when she got home and say, "aaaauuuuunnn" because he knew she wouldn't put him in timeout for it.
So my biggest piece of advice would be that both you AND your husband must sit and tell his sister and him there is a new discipline in the house. That both of you are going to be doing it, and everyone in the house is going to go along. It'll make your home a happier place for all of you. You all should sit down together and make house rules (make sure you talk about how arguing hurts families, and anything else that is disrespectful). And then BE CONSISTENT. It's hard, especially if you're tired, but it only lasts 1-2 weeks and then you'll have paradise!
Try your best and be firm and consistent. Always give him a replacement behavior (especially since he has ADHD). Don't have him sit in timeout, make it something like standing in a corner or tape up a 3x3 square aread on the floor so he can move around (he may be unable to sit still for 8 minutes).
You can tweek this a little, but not much.
Good Luck!