Yes, this is familiar.
You are a good mother, but your parenting methods, which are perfectly OK for most kids, are the disaster here for this kid. Not your fault. Not anybody's fault. You are not wrong, but you need to be more right for this situation.
You say he does fine at his fathers, because not much is expected there - you are dead right. But it is also becoming a bigger part of your problem.
IS his baby sister's arrival part of the problem? Maybe. Or maybe he is just getting that little bit closer to teenhood and its hormones, and that is coming out.
You need to change how you handle him, and there are easier (and better) ways. Think how the school has to handle him - a lot of what you try to do at hoe, things we have been taught are a parent's prerogative - the school is not permitted to do. But they manage. You need to find out what is working for them and begin to use it too.
We tend to think that our difficult children need a tighter rein simply because they are difficult children. Often they need a looser rein, and supported instructions instead of extremely tight controls. To begin with, you need to let go a lot. Almost everything. Let the reins go completely slack. Then slowly you pick up the slack and stop BEFORE you reach the point where he begins to fight the rein. If you get to the point where he is fighting the rein, slack off again and wait. Try again. It takes patience and calm, but the rewards are worth it. Over time as he learns to trust you the reins can tighten, but never as tight as they were before. Instead, you have to teach him to do for himself, the control you want to impose.
Avoid blame and "fault". Instead, bring in responsibility. For example, we stopped making out kids tidy their rooms. But any of their stuff in family space - it's got to go. If their room was so bad that nobody else could get in, then the consequences are - no washing gets put away by parents. No washing gets collected by parents. No tidying happens from anyone else. The child is responsible for his own space. And when our kids broke the rules and took food into their rooms, we found the resultant ant plague was so unpleasant tat they HAD to tidy up in a rush. I didn't put the ants there. But leaving sweets lying around in the bedroom (even one dropped jellybean) was enough to make their room very unpleasant. Natural consequences. And avoid "I told you so" because with your son, that feeds into his need to always have the last word. Instead, he needs to see it for himself. It has to become "I told me so."
Try to put requests and instructions in positive terms. I agree, throwing a ball inside is dangerous. But this is a kid who probably hears "no" and "don't" to excess, so it loses effect. Instead, say "Please take your ball game outside where it is safer." It is a positive instruction, giving him something to do, rather than something to NOT do. The outcome (should he obey) is the same for each instruction. Plus he has something to do, to continue with. If he has to stop, he needs a new activity to move to. CHanging task is often a problem in DHD as well as other disorders, and I suspect this is also a huge conflict area for you. You need to be aware - he is often not like this purely to be difficult, but because it is something very difficult for him. Again, I suspect his father is instinctively giving the boy his head on this, too, by letting him continue what he is doing until he moves on by himself. Again, you need techniques. What worked for us (computer gaming is always the big one, they hate to stop gaming and, frankly, games are designed to keep the player hooked in and wanting to continue) was asking him how much longer he needed. Or giving him plenty of time. "I need you to come eat your dinner in half an hour. Get your game to a save point, or pause it, in the next half hour. If you don't, we will eat dinner without you and your plate will go cold."
The same applies to bath, to chores etc. With short chores, we often ask the difficult child to pause his game for long enough to get his chore done. If difficult child 3 is watching his favourite TV show, I sometimes ask him to complete a chore he can do while watching TV (such as putting away the washing up). Or I ask him to to a chore in the ad break. He feeds the chickens, and I stand at the door to call him if the ad breaks finishes sooner. He can finish the chickens next ad break. I help him not miss his show, he then gets his chore done more cheerfully knowing I will be there to call.
Another angle you can try, is to reward him for a period of time not arguing about his chores. You have to set this up with him ahead of time, and negotiate a suitable reward system. A type of token system works well, especially if the reward is non-material. The gift of you and your time with him, spent doing something enjoyable together, is actually a gift for both of you (but don't tell him that). For us, the reward for difficult child 3 not arguing about his chores or having to be sent to his room for meltdowns for one day, was 15 minutes of me playing Mario Party with him. difficult child 3 would accumulate his minutes and on the weekends we would sit down to hours and hours of Mario Party. It is important to make the time available to cash in the reward, or it stops working.
You have a fledgling lawyer there, so use this to your advantage. Discuss with him, don't argue. Ask his opinion. For example, when I was certain that difficult child 1 had no idea how much time each day he spent gaming, I asked him to set his own limit. "What do you think is a reasonable amount of time per day to be gaming?"
difficult child 1, grinning, thinking he had me conned, said "Three hours."
I said, "Done! You can spend three hours, max, playing your games. The rest of the time you find something else to do, including your schoolwork and your chores. Any other spare time, you can watch a movie, go for a walk, go for a swim, read a book."
difficult child 1 realised very quickly that he had underestimated his gaming time. This made him realise how much time he was losing to gaming.
If your son has any aspects of Pervasive Developmental Disorder (PDD) in there, he will also tend to follow rules well. At the moment they will be the rules as he believes they should be, but he is still likely to be following some sort of rule, nevertheless. What you need is to show him that your rules are right, and should be followed. This requires you to show him how they work in his life, and work to his advantage. That will take longer.
A book that should help with all this, is "The Explosive Child" by Ross Greene. It has helped a lot of us with this sort of problem. But we also here have a lot of our own experiences to share with you.
You will find what works for you. You and your child have to find your own level. But it should work out. The person who needs to change first, is YOU. Again - this is not about fault, it's about who has to start first. That is all.
Welcome, glad you found us. This CAN work, and should work well. But it won't be all smooth sailing, and he will still have ADHD. But he will be closer on track to learning how to manage it for himself, instead of relying on you to be his keeper. You need to now work on being his facilitator and not his warder. You will be happier, and so will he. ANd this will heterodyne positively.
Marg