I wouldn't call Bro-Sis vibe between actors when they are enacting romance onscreen. I infact see it as demeening and ridiculing the pious relationship of Bro-Sis.Co-stars are not Bro-Sis and neither enact that onscreen. I would definitely say no or less chemistry or great chemistry.
I said Varun-Alia give me that vibe not because of onscreen but offscreen, the way Varun sang "pholoon ka taaaron ka" for Alia on KWK and "You are my pumpkin" on some award show and called him his sister in an interview to which Alia responded him with a bhiya. Those are my reasons that they treat each other like Bro-sis offscreen. They had a good chemistry onscreen both in SOTY and HSKD. Different movies show different kind of chemistry like lover's or friendly, silent, hyperactive, violent. I can only say no chemistry or good chemistry onscreen or how they behave offscreen(brotherly-sisterly)
Like Ranveer continues to insist how he treats Priyanka as his sister offscreen (though Priyanka doesn't like it)and you automatically start relating her as his sister relationship offscreen and hence lose interest on the chemistry they show as couple onscreen.
"No chemistry" i'll go movie wise as they are good in some and bad in some.
Ranveer Anushka in DDD(bad)
Ranbir-Anushka in BV.(awful)
Kareena-Salman
Varun-Shraddha (I love Varun but they were bad in ABCD2)
Aamir-Katrina(Dhoom3)
Imran-Kareena(EMET)
A lot depends on script. I'll take example of Sid as he is my fav. In SOTY the chemistry he had with Alia was great because it was forbidden. Chemistry rocks when it's forbidden love.He was not supposed to eye his frien'd gf but couldn't help being attracted and internally silently nurtured that feeling while controlling it externally. As "Apple of My Eye" said he had that "f*ucking with eye" chemistry with her which meant a lot of suppressed passion he had to enact which always makes for a great chemistry. In HTP my all time fav rom-com...he had very good friendly banter and easy chemistry with Parineeti which is again a great chemistry.
Ranveer's friendly chemistry with Parineeti in LVRB was awesome but their passionate or lover's chemistry in Kill Dill failed.