A sister republic is like having a group of friends who are very similar to you and you work together to achieve things. It's like having a sister who lives in a different place but you still love each other and help each other out.
In history, a sister republic was when one country would help another country become more like them. For example, France helped create sister republics in other countries during the French Revolution. These countries wanted to have the same type of government, laws, and ideas as France.
So, France helped them create their own republics that were similar to France. Each country worked together to make their government better and to help each other out. Just like how siblings work together to make their family stronger and help each other out.