Some of it might just be their current ages. Mine are 9, 8, 6 and 4.5. We go through phases when one pair or another just doesn’t get along as easily and it is usually a phase. Ages that are a multiple of three tend to be extra tough. That said, it might help to identify what you are hearing at a neutral time. They might have some insights themselves about what some of the specific issues or triggers are. You can also model scripts or role play for how they can handle those situations differently.
Another idea is to try to find things to do as a group (you too!) to reset those relationships and give them something positive to work off of. Sometimes we fall into bad habits of relating to each other and need to be reminded what kindness and graciousness look and feel like.
My kids think it is great fun to name three things they like about each of their siblings (sometimes Mom and Dad too!). It makes for a fun mealtime discussion. Another fun activity is to get a journal like
this It says it is for three years, but we just assign each kid a space on the page and enjoy hearing each kids’ answers.
It is so stressful when relationships are not as smooth as we would like