Well...my girls are 3 and 4 (almost 5 though). As soon as the youngest was able to move around and take things, I had to constantly be there. Their impulse control is horrible when they are babies and toddlers. When I saw one of them grabbing at something the other was holding, I would physically grab her hand and say, "Oh, W's playing w/that right now." I often try to offer her something else to play with or as DD2 got older I would mention that she can choose anything else to play with. If she REALLY wanted the toy (as they always do when someone else is playing with it lol), I would model what's the proper way to ask for it (at least in our home lol), "W, please let me know when you are finished with x, y, or z." Amazing...giving them that script to speak seems to help them w/their impulse to reach out and grab...even when this started years ago. Most times the other girl will say "Ok." When they don't respond and the other is getting upset I will say, "W, did you hear what M said to you?" to get her going.
You see, we don't force sharing. I don't even force taking turns...ie...setting a time limit before one of them has to give up the toy to the other. Basically, b/c I didn't want to always be there to be that referee lol. I feel if someone's playing w/a toy...they can play for as long as they like. The other kid just has to find something else to play with. Of course, I'm not always too strict on that. It really depends on the object. There are certain loveys, blankets, and shoes that each girl has that if the other is playing w/it when the original owner wants it...I make the owner ask for it and then I will add a line after that, "Oh M, W's really happy to be able to play with YOUR monkey. I'm sure she will give it to you as soon as she is finished with it (or as soon as she finds something else to play with)." And I'm not too strict on that when we go out...like we (ok, I) have a set time limit at a place and they both want to ride on something.
When I hear one of them complaining that the other isn't playing the way that they like, I just say, "Sweetie, you can choose what YOU want to do."
My oldest is quite the reminder to my youngest about 'how things are supposed to be in this world' haha. The line I ALWAYS say (ugh - still have to use this lol) is, "Oh W, please take care of your responsibilities." And when DD1 comes running to me b/c DD2 is doing such-and-such I usually say, "Oh, that's interesting. Now...tell me about YOURSELF. What have YOU been doing?"
That's is what the other doing was something harmless. Sometimes (if *I* have the patience lol) I will talk more with her. Like say if W came to tell me that M was calling her dopey (ugh - for some reason they just LOVE the name of this dwarf and it makes them crack up....only if they are both in the mood though lol). Then I'll say, "She was calling you that? Oh, you didn't want to play like that right now, huh?" "Well, what should you say to her? Remember, 'I don't like that! Please stop!" Sometimes that will work. And sometimes if that wasn't enough I'll tell her, "Well, once you tell her that you didn't like it and she did not stop, then you need to go away from her and find a nicer place to play." And if the other still bothers the other, then I have to step in to say something. And I've also told DD that if all those don't work (expressing in words that you don't like what's being done to you and moving away) to go tell the teacher or a grown-up. Basically, I want them to try to work it out first...before bothering (lol) me.
"I don't want those racecars!" I try to model "No, thank you."
As for legos and books (my oldest loves to flip through books and would often stack 10-20 books next to her on the sofa lol), I give them a fair amount to play with...then the rest is up for grabs. Like for books, even though DD1 stacked the books next to her...if they start arguing about it I usually say, "one at a time."
And that's the same for toys. If one is playing w/something and builds something...and then leaves it to go somewhere else or play w/something else....well, it's up for grabs for anyone. I usually say something like, "I'm sorry, M made sure that no one was playing w/it when she picked it up. If you would like to play with it...you know how to ask for it (the "Please let me know when you're finished." line lol). Some exceptions I make are when I've made one of the girls drop their toy to do something...like go to the bathroom lol. Then I will say, "Oh sorry M, W was playing with that when mommy told her to do x, y, and z. Please give it back to her and she will let you know when she is done."
And sometimes when they get really rowdy (not fighting, but they just sometimes get so hyper playing with each other) I will give them one warning that if they can't play more calmly that we'll have to sit on the couch (separate couches) and quietly read books (one book at the time). Yeah...usually it's not b/c they are fighting but that *I* want some more peace. I know, I'm bad lol.
And basically I've told them (DD2 probably doesn't really totally understand yet) that there are 2 rules when we are out (either in the living room and even when we go out): we treat people respectfully AND we need to be safe. I am your mommy and I will make sure that we are safe at all times. If one of them is doing gymnastics on the arm of the couch, I will say that, "The couch is for sitting. If you don't know how to use things correctly then you will need to rest on your bed." Or if it's w/how they are playing with a toy...I will say that if the toy isn't being safe then the toy needs to rest and we'll all try again another time.
And some other things you didn't mention, but I'll mention b/c it gets on my nerve haha.
"I don't want to do x, y, or z!" = I will say something like, "Oh I can see that you really don't want to take a bath because you are playing with your princesses. Problem is that it's bath time, so you can take a bath happy or sad." So far this has worked...often times they will add their own like "silly, quiet, pretty, *noisy* (*sigh*...when talking about how they want to be for their nap/sleeptime lol).
"I'm bored!" = I say something like, "How funny that you want to be bored when there's x, y, or z around to play with or do." or talk to her about how she's in control of what she picks to do, she needs to decide. Sometimes I tell her that you can't always pick where you, but she can pick what she wants to do. Sometimes you can't pick what you are doing (bathing, running an errand, etc), but you can choose how you are going to do it (happy, sad, etc).
*my* biggest fault = yelling to them in the living room from the kitchen, "TALK NICELY, PLEASE!!!!" haha *sigh*