OMG

I guess she didn't tell her feelings because of her problem not because of Tsubaki or anyone I guess(completely my opinion) she realize her real feeling a bit late and didn't want to trouble him

I know she didn't want to trouble him because of what happened with his mother and death is always hard to deal with especially if its someone you love..but I think she also held back because she knew her friends feelings troward him..she knew she was going to died why hurt her best friend. ...she was just happy being by Arima side.

Me too

I agree... I love Kaori and can't like Tsubaki no matter what T^T
Kaori probably didn't want Arima to be even more attached to her and then end up depressed again because she died. Plus, she really did like Tsubaki so...