Mortgage and working for family in new start up. Problems?

Hi there, my sister has recently started a company within the last few weeks, she's investing a lot of money into this business and she is about to take me on as an employee of this business.

I'm going to be leaving a very well paid job for which I'll be earning about 15k extra a year working for my sister earning well in excess of £56k a year..

I plan on getting a mortgage in the next 8 months or so, will there be any issues getting a mortgage in this new job working for my sister even though by then she won't have a year's worth of accounts with hmrc?

I won't be paid cash in hand it will be through the books now I don't know the ins and outs of managing a company but she said she has a meeting with a payroll company on Friday to arrange managing my pay/ payslips and tax to HMRC and any future employees she eventually takes on down the line.