Ah I have the same problem. Never have confidence that my expectations are fair and then DS plays up to it. I think you need to decide what your expectations are in advance, discuss them with DH and then both stick to them - and keep your expectations consistent. Don't worry about what is fair. Worry about what you are expecting. If the expectations start to feel unfair or inappropriate, which of course they will in time, then you and DH can talk again and adjust them to something you feel is more appropriate. As they get older, it will be easier for your expectations for both of them to be more similar.
For example, if you've decided you expect them to put away toys which aren't part of the current game, you can't one day let them get everything out and ignore 90% of it, and say/do nothing, and another day get on their backs because they've got three things out at once which are all involved in some complicated game. It doesn't really matter if you address the issue of the toys being out in different ways depending on your mood/the situation etc, it's just important that your expectation is always the same. (And it can include exceptions like "Only one toy out at once when we have Great-Granny round" as long as you explain that it's a change from the norm and if age appropriate, why.)
Same with bedtime - set out your expectations with DH in advance, and of course they can be different for each child based on their age and personality, but then stick to them. It doesn't matter that DD2 doesn't know what it is to be an only child, she has a big sister, that's who she is. There's no point thinking "Oh, what did we do when DD1 was this age?" because they're not the same person, they have totally different needs and reactions even as babies. What's being fair to them is treating them as individuals, and not agonising about having to do things in the same way all the time. You love them both the same, and yes at times they will feel that you're being unfair about something, but that's just life - and part of being a sibling!
The other thing as well is that it's important to make sure your expectations ARE heard, which means that if they're not listening then you need to try something else. It's good to set out expectations in advance and often they then just need a reminder. If in the moment it's not happening then you can make it happen which might be by helping them or might be some kind of related consequence like "You can tidy these up or I'm going to tidy them up but they're going away for a while". I don't like generic punishments and try to use something which teaches the lesson as a primary point rather than hurting as a primary point, but at 3 you will get the boundary pushing and sometimes it's necessary especially for behaviour like rudeness or physical violence where you can't really use something related - I wish I'd realised this earlier. I use something really token and non-frightening, in our case screen bans. Of course you can also deal with the behaviour in an empathetic way as well, but I found that actually despite what all of the "gentle parenting" sites say I just needed something to put on top to say "Actually, no that wasn't OK." because me saying "I won't let you speak to me like that" kind of still was letting him speak to me like that, and I wasn't about to hit him back.