It would count as a spin-off if your sister is the exact same character in 2 stories, but you can make a character that will look like her, but she has to be a brand new character with a new personality and background.
Like for example, in one of my stories my friend, Sophia, is a fairy on another Earth, but in another story, she’s a secretary XD but lol that’s a bad example, cuz the other story is actually a spin-off XD
Anyways, my point is it’s okay to make a character look the same, but change their experience. And I’d recommend altering the name too. For example, Sophia/Sofie, Maria/Marie, etc