Is chibichibi related to Serena and Rini?

it depends, in anime she is not, she is Galaxia's star seed, but in manga she is future form of sailor moon and in the end she changes into sailor cosmos

Personally I'm not really sure, because right now I'm rewatching all the episodes, and I'm currently at Sailor Moon R (part 1). I kind of thought she was Serena's granddaughter, but that's just me!!!

i'm just guessing, maybe she's rini and helios daughter! :D

In the anime, no, not at all. She's just the Light of Hope. How cheesy, I know. In the manga, she has a much better story. Chibi Chibi is Sailor Cosmos disguised. Sailor Cosmos is Usagi from a parallel future, a doomed one at that, and she returns to the past to try to get Eternal Sailor Moon to destroy the Cauldron, where Senshi are born. This would keep evil from being born, but it'd also keep good Senshi from being born, so Usagi refuses. Cosmos decided that Usagi made the right choice and goes back to her universe to confront Chaos. Cosmos is also 30th Century Chibiusa's actual mother. The Neo-Queen Serenity we see in R and the 2nd arc is Cosmos before becoming a Senshi again. Our Usagi has her own Chibiusa in the future as well as a 2nd duaghter, Kousagi. Hope that wasn't overkill^_^