In Dragon Ball Super, Kale is calling Caulifla all the time "ane-san", which means "big sister" in japanese, but in the other hand Caulifla talks about Kale as her disciple, and I dont remember she ever called her sister. Are they sisters or Kale call Caulifla "sister" in an affective way?
No, i don't think they are sisters, as Calulifa said she just recruited Kale. When she first met Cabba, he asked who she was and Caulifla said she is also a saiyan. Also if he was her sister Cabba should at least have heard of her since he knows Renso well.
Regarding a definite proof there isn't one and most likely there will never be one. Usually, through story telling we are almost always told if some characters are blood related, so if no such info comes out in DBS the conclusion is obvious.
No because kelfla and broly can both turn into a legendary super saiyan and kelfla appeared a little while after broly was defeated