It's tough at that age - there was a four year gap between my girls and DD1 at least was very well behaved, so it was never quite as bad - but even today the bickering still drives me mad. My two nieces have the same age gap as your boys though and they can be very hard work. They tend to be OK for me, but not so much for their parents and nan but I parent them the same my I parent my own kids when they are with me.
BIL and SIL tend to go a bit more down the gentle parenting route (with the occasional gasket blowing) and have constantly done stuff with the kids, so they are used to being permanently entertained, which means their behaviour can really kick off when they are not doing something.
Our parenting style is much more along the benign neglect / pick your battles line for most things (like food, or helping them get dressed etc) but the three things we were incredibly strict on, and set expectations very early was an insistence on politeness, zero tolerance for non-consensual physical fighting or verbal teasing and behaviour in public (no screeching or inappropriate shouting etc) - all of these were met with swift rebuke and consequences, but because we were really relaxed about everything else, they knew that when we got cross over these things, we meant it.
I remember being on holiday with BIL and SIL - they just had the one 2 year old boy and my youngest was 18 months - I was outside the house and I could hear my youngest crying - there was a steep stone step into the kitchen and I was worried she'd fallen. BIL was holding her and when I asked what happened he said that she'd bitten her sister and when he told her off she'd started crying. He was really surprised when I took her off him and marched straight upstairs with her, sat, her on the bed and told her she was in a time out for biting. She could sit there for two minutes and then apologise to her sister and if she didn't apologise she would sit there until she did. He was shocked when I went back downstairs and said she was in a timeout. It worked and she never bit her again.
For things like getting dressed, putting shoes on etc, I turn it into a game - use their natural rivalry to help you and join in "Right, time to get dressed - whoever does it first gets....<insert small treat of choice>" or combine with other things "Whoever gets their shoes on first gets to open the door when we get back" and for whinging and whining, I use to use distraction techniques like playing eye spy or completely ignoring the one that was whining and having a fun conversation with the one that wasn't.
I also used to tease them back - so if they were moaning that they were too tired to put their shoes on, I'd do it for them but as I was doing it I would say things like "Aw - do you need me to put your shoes on like a little baby", or if they were being particularly difficult and I was getting really annoyed "Well if you are too tired to put your shoes on, we'd better stay in and you can go back to bed to rest"