This is called “social play”. It is the way kittens interact with each other. It is different to how children play. Kitten playtime is based on fights and predation (hunting animals) whereas although children sometimes fight their play together is generally cooperative and not agonistic (genuinely combative). The fact that kittens fight-play and play-hunt clearly indicates that the raison d’être of the domestic cat is to be a solitary hunter. It is hardwired from conception. The human is not hardwired in this way. Although you’d be forgiven for believing it, based on a lot of human behavior.
Kitten play is not full-blown fighting but limited by the complaints and avoidance behaviours of the cat getting smacked or bitten. Also facial expressions provide a signal that the encounters are playful and no agonistic.
Between the ages of four to seven weeks three or more kittens are usually involved in play sessions. As they grow up they are more likely to end up playing in pairs. Social play peaks between weeks nine and fourteen and depends on the opportunities to play.
Genuinely aggressive encounters creep into kitten play at about ten to twelve weeks of age. Where a litter is keep together, at around fourteen weeks of age social play declines. It is replaced by “object play” which is playing with small objects and toys which helps develop eye-to-paw coordination.
When kittens are weaned and placed in their forever home at eight or nine weeks of age they play with their human companions, as we know, unless they are adopted together.
This is a video I made years ago and which I often use. It is well suited to this page. They are an F1 and F2 Savannah kittens play-fighting really hard.
A question is whether siblings get along when they become adults. I was told by a cat rescue woman (from whom I adopted my current cat) that in her experience siblings don’t get on. She was partly wrong. It seems that it depends on individual personalities. Sometimes they get along and sometimes they don’t carry forward into adulthood their friendship during kitten hood. Perhaps this reflects the development of their characters to the point where they clash or bond. If two sibling rescue cats get along they should be adopted together if at all possible both in the best interests of the cats (they can entertain each other) and for the benefit of their human guardian (less demands on entertaining them).
Some pages on kitten behavior