Almost certain your sister will need a family visit visa, as been in that situation myself last year. details i wont go into. But basicly if the intended applicant is a family member uk immigration rules are they have to apply for family visit visa if intended applicant is staying with the family member or visiting.She should get visa if she can convince ECO that she will leave after her visit, and if she has a good job in dubai like you said she should be able to. because that being the thing that most family or visit visas get refused on by ECO(leaving uk after visit).........