Here's why:
* Sailor Chibi Moon is Usagi Tsukino's daughter from the future. She is a reincarnation of Princess Serenity, the original Sailor Moon.
* Usagi becomes Sailor Moon. She is the protector of the Earth and fights for justice.
* Chibi Moon becomes Sailor Chibi Moon. She is a powerful warrior who joins Sailor Moon's team to protect the Earth and save the future.
They are connected through reincarnation and time travel.
Chibi Moon travels back in time to meet her mother Usagi, and they join forces as Sailor Moon and Sailor Chibi Moon. However, they aren't meeting as strangers, they are already family, just separated by time.
To clarify further:
* Usagi and Chibi Moon are mother and daughter
* Sailor Moon and Sailor Chibi Moon are teammates
* Princess Serenity (Sailor Moon's past life) is Chibi Moon's past life
They have a unique and complex relationship, which is a central part of the Sailor Moon story.