Is it ever okay to have a completely different standard for one child than all the others in a household?

I'm considering moving myself and my sons (5 & 7) in with my partner, who has 3 children, two biological sons (12) and an adopted daughter (15). All the boys would be sharing one room. The daughter has her own room. 

The daughter has completely different house rules to the other kids.

1.She has a lock on her door, which the other bedroom doesn't have.