How Can Fighting Be Healthy?
I remember growing up and fighting with my sisters a lot. We would fight about some of the most ridiculous things ever, between who gets to pick the next television show to 'stop wearing my clothes or I'll rip them off of you.' But once the name-calling stopped and the hair-pulling eased up, we would be back to playing barbies and laughing together. There is a reason why we fight and why kids fight more frequently than most adults.

Disagreements among children is very common. Typically a disagreement occurs when a child tries to figure out the world through their lens. This happens when children may feel they are being mistreated or when establishing their rights. For example, a child may begin fighting with their sibling who took the toy the child wanted to use in their play. A simple mishap such as this can cause a huge emotional outburst.
Neurobiologically, your child is still learning to identify, express, and manage their emotions. It is unrealistic for us to expect our children to manage their emotions when they are upset if they haven't learned how to. As adults, our role is to help children learn. Fighting can provide valuable lessons and maybe shouldn't always be stopped or hidden. (I give this feedback with the recognition that every child is different, and you will need to assess your own child's needs. But also remain mindful that children are resilient and can be challenged.)
Unless someone is going to be physically harmed, children often don't need a referee keeping an eye on every played move or dishing out consequences when we see it as unfair. Children need someone who will provide a safe environment where they can explore their emotions and a mentor to gain guidance. Fighting helps children learn to problem-solve and gain the necessary social skills needed for when they become adults. As they fight, children will learn to negotiate, see another's point of view, and learn to respect other people's rights and feelings. As time progresses, fighting will decrease.
Awareness of how we, as adults, fight with others is also a valuable lesson children learn. When a child sees their parent's fighting or senses the after-effects, they are learning how to fight, how to empathize, and how to forgive. Children do not need to be aware of the reason for the fight (that is an adult conversation and should be discussed away from the child), but they can see, hear, and feel how you and the other adult are fighting. Allow your child to explore and understand the situation while simultaneously providing a sense of warmth and comfort in the environment. When you do this, your child will begin to mimic these behaviors in their disagreements.

It's a lot harder to hang back and watch your child fight with another child when you want to step in and provide them the answers. It's also a lot easier to tell your child your frustrations over your own personal disagreements. But by challenging yourself, you teach your child how to respect and empathize with others and themselves.
Fun Activity
Despite the sibling rivalry, to build that family attachment, grab a balloon and blow it up about 1/2 full. Each family member gets in a large circle. Pass the balloon back and forth without it touching the floor. Add a challenge by using different body parts (i.e., elbow, knee, head). This is a great activity to focus on teamwork and family attachment.
The new year seems to offer a glimmer of hope that maybe things will start moving in a better direction.