Is "workplace senority" justified, abused, or a little of both these days?

I work in a retail establishment in a department with 3 other people. Two of them have as much time as I do with the company (all of us 3 years), but I haven't always been in my department. One woman seems to think that because she has always been in the department and has been with the company 1 month longer than the other woman (who has also been in the department since the beginning, then she can have first dibs on everything. This includes (not all of them) picking her lunch first, what part of the department she wants to work in (we have 3 sections), and picking the two days she wants off first (schedules are set, but are reviewed yearly). Can I get some opinions and possibly some examples of experiences and your thoughts on this? I am going to pick the answer personally so its an easy 10 points. Thanks :)