Of course, you need to assure that the children are safe. See sibling relationships as an opportunity for children to learn how to relate to each other. Many of us grow up without knowing how to fight fairly or without demeaning one another. We used to think that very young children didn't engage much with other children. We realize now that they can do so in a sophisticated way by 11 or 12 months.
Toddlers are capable of much more complex play.
They are learning the give and take of games, to share, to negotiate, and to construct elaborate dramatic sequences. Friendships are important to young children. Children like to do the same things over and over, they can sustain these activities long after an adult becomes bored, they teach each other new skills, and they inspire confidence. Patterns of relating to friends get established in the early years, so it's important for young children to experience other children as this is the time that is ripe for learning. Children who learn to play with other children early are more likely to get along with other children as they grow.