Upvote:2
According to Family Visitor visa - Documents you must provide the only thing you might need to provide from your employer are pay slips.
In particular, what they're wanting is proof that you have "enough money to support yourself during your trip, eg bank statements or payslips for the last 6 months".
From your sisters side you'll need the following:
Seems to me that it might be easier to apply for a General Visitor visa, however, and say you're visiting a friend instead of a family member idk.