Sylus’ pet name?

I changed my nickname for Sylus to “Darling” and he said 姐姐 (My game is in Chinese)

I speak Chinese but didn’t grow up in the environment, so I’m just wondering

Is it a neutral term of endearment between people you’re close with? 姐姐 when used in a normal context is for people older than you, but if it’s a term of endearment can it be anything? XD Sylus is 28 and I’m nowhere near that age lol