While 'Isn't Julia anyone's wife' could be 'क्या जूलिया किसी की भी पत्नी नहीं है?', you can still use 'anyone' whether or not you're asking a question in the negative; e.g. 'Is anyone here?'
To me, the phrase 'Is Julia anyone's wife?' sounds at least as natural as 'Is Julia someone's wife?' - are you suggesting that to mean 'anyone' would require a negaitve in Hindi or that 'किसी की भी' is required to denote 'anyone's' rather than 'someone's'? If the latter than that would make sense but the addition of 'नहीं' seems to completely change the tone of the question:
a) 'क्या वह किताब आापकी है?'
b) 'क्या वह किताब आापकी नहीं है?'
कोई can mean both something and anything and similarly, किसी can mean both someone and anyone in most contexts. By adding the भी, we specify that we mean anything/anyone rather than something someone. (Actually, कोई भी is a lot like 'anything at all').
There is no particular reason I used a negative sentence in my previous comment and you can just as well use क्या जूलिया किसी की भी पत्नी है = Is Julia anyone's wife?