I have two girls, age 7 and 3.
7 year old has her silly moments but by and large is quite gentle, thoughtful and sensitive.
3 year old is a force of nature I am just barely controlling tbh. She is lovely but terribly strong-willed, and given to kicking off if she doesn't get her own way.
The girls by and large love each other very much and play really well together. They both understand in principle what compromise is and how it can be useful.
But 3yo is 3 and every now and then we will hit a situation where she is just going to strop.
Yesterday's example: I take 3yo with me to pick up 7yo from ASC. It's raining so give 3yo her brolly and bring 7yo's with me to give her. The second 7yo opens her brolly 3yo goes mental because she wants THAT one, not her one. 7yo doesn't want to swap because 3yo's is much smaller. Both crying at this point. I back up 7yo and remove her brolly from the 3yo who has snatched it, with great difficulty and very embarrassing in front of all the other parents with their suddenly impeccably behaved sprogs :P 3 yo screams her head off and within seconds 7 yo has handed her brolly back to 3yo and is wailing herself.
This happens with all sorts of things. Kids disputing, 7yo has the right of it, I back her but then 3yo throws off and 7 yo undermines me by giving 3yo whatever she wants whilst being very upset about it.
She says it's because the noise of her sister crying is worse than not having the thing she wants. Which is sweet and all very well, but it still means I have at least one crying child! And I'm worried (a) that 3yo will absorb the idea she only has to kick off long enough and loud enough to get her own way, and (b) 7 yo needs to stick up for herself more or she's going to get walked all over in life!
Any suggestions? I mean obviously I could (try to) separate them so 7yo doesn't have to put up with the tantrumming 3yo, but not always practical.