Depends on what you mean by "other ending".
Early ending 1 requires mocking Rikako in chapter 2 and taking Arisu's offer of a quiet life in chapter 4.
Early ending 2 is a bit more complicated. It requires not being on any of the main girls' (Sarah, Emily, Alex, Stella, Arisu/Rikako) paths by the end of chapter 6.
As for Fuji sisters, for now it's either Ari or Riri. Whether there will be an option to have both or not - only HanakoX knows.