My kids have been fighting a lot without any reason. They have reached that stage where everything turns into a rivalry. I don’t want to be a strict parent but need to set some ground rules and boundaries so please give me good suggestions.
Oh man, I feel this so much. My two went through the exact same phase last year and it was exhausting. Like, they’d fight over who got to sit in the good chair at dinner. The SAME chairs they’d been sitting in for years!
What worked for us was giving them each their own space and their own stuff. Sounds simple but it made a huge difference. When my daughter knew her art supplies were hers and her brother couldn’t just take them, suddenly there was less screaming. The rivalry dropped way down once they weren’t constantly fighting for our attention. 
Hey, so this might sound weird but hear me out, let them fight a little bit? Obviously not like physically hurting each other, but some conflict is actually normal and teaches them how to work things out.
I used to jump in and solve everything for my kids and it just made them more dependent on me to referee every little thing. Now I only step in if it’s getting mean. Otherwise I let them figure it out.
They are surprisingly good at it when you give them the chance. They get frustrated, and then they move on. It’s part of learning to deal with people who don’t always agree with you. Just my experience though! 
You need clear rules and you need to stick to them. That’s it. Kids push boundaries to see what they can get away with, and if you don’t have firm lines, they’ll keep pushing.
Um, I don’t have kids yet but my younger siblings fight constantly and I’ve noticed a few things that help? Maybe these could work for you too…
Like when my parents give them tasks to do together instead of separately, they actually get along better. Something about working toward the same goal makes them teammates instead of enemies. Could try that?
Also my mom started having them say one nice thing about each other before bed and at first they HATED it but now it’s kind of sweet. They still fight during the day but it ends on a good note. Just a thought! 
The thing about sibling rivalry is it’s usually not about what they are actually fighting over. So addressing the surface issue doesn’t really fix anything.
I read somewhere that kids fight more when they feel like love is a limited resource. Like if one gets attention, the other gets less. Once I made sure both my kids got quality time with me individually, the competition calmed down significantly.
Also, stop comparing them to each other. Even positive comparisons create rivalry. Why can’t you be neat like your sister? that stuff fuels the fire.
Just gonna throw this out there… have you checked if one of them is actually being treated differently? Because kids pick up on that stuff instantly and it breeds resentment. My parents definitely had a favorite (not me, thanks for asking) and my brother and I fought constantly because I was angry about it and he was defensive. Took years to repair that relationship after we both moved out.
Make absolutely sure you are being fair. Kids have a radar for unfairness and it makes them turn on each other.
Okay so this is gonna sound harsh but maybe they are fighting because they’re bored? Kids with nothing to do turn on each other for entertainment. Get them involved in activities, hobbies, sports, whatever. When they have their own interests and friend groups, they don’t have time to constantly bother each other.
And yeah, screen time limits help too. I noticed my nephews fight way more when they are both stuck on devices and then someone wants the charger or whatever. Give them actual things to do and the rivalry becomes less intense. 
What helped in our family was having designated yours, mine, and ours categories for stuff. Some toys were shared, some belonged to each kid individually. When they knew what was what, there was less fighting over possessions.
And look, sometimes kids just need to be separated for a bit. If they are constantly in each other’s space, they are gonna clash. Give them separate activities Distance makes the heart grow fonder and all that. 
I went through this exact same thing last summer and it was brutal. Every single day was just constant bickering and tattling. Drove me absolutely insane.
What finally worked was family meetings where everyone got to voice their complaints and we came up with solutions together. Sounds cheesy but it actually helped because the kids felt heard instead of just being told what to do.
We also implemented positive time where for 30 minutes a day they had to do something nice together. At first they hated it but eventually they started actually enjoying that time. Now they ask to do stuff together without being forced. Wild how that worked out!
@VoipMax, You know what nobody talks about? How exhausting it is to constantly referee sibling fights. Like, I signed up to be a parent, not a full-time judge and jury.
Well, my advice to you @geekandgamer is to pick your battles. Not every fight needs your intervention. They are fighting over who sits where in the car? Let them figure it out. Someone took someone else’s pencil? They can work it out.
Save your energy for the big stuff. Everything else, they can handle. You are not being a bad parent by not solving every problem. At least that’s what I tell myself when I ignore the 47th argument of the day about absolutely nothing. 