Dependent on the circumstances there may be ill feelings though this is sometimes inevitable when you go in, in a position of any authority; perhaps the people who you could end up managing feel one of them ought to be in the higher position. Or they may have a loyalty towards the person who is ill and therefore you could be replacing.
I would be concerned that the store have said to you if you make a good impression, the role could go perm. I guess it depends on the severity of the illness this person has but if they get better, then they should be encouraged to return to their role, regardless of how you're getting on as cover.
Any how, if you've no better options, I would go in anyway and see how you get on. You would like to think your colleagues/sub-ordinates wouldn't purposefully make your life difficult, but you never know. Just be as nice and fair as you can. If they do start to turn on you, make a point of raising it with your manager.
As to the person who is ill, ask the appointing manager or the HR department what the score is there; they shouldn't go into detail about this person's illness (confidentiality) but you have a right to know what kind of period of time you should expect to be required for. Once you go in (if indeed you choose to), the people you work with may know what's wrong with this person and divuldge that to you unofficially, so to speak.
Good luck!!