Buy out from joint mortgage with sibling

Hi all,

Hopefully this is in the correct forum as would be great to hear of anyone who went through similar experience.

My sister and I bought a 2 bed apartment together in 2017 and I recently moved out to move in with my boyfriend while she agreed to essentially rent out my room from me as her home office as she's still fully working from home. Now she wants to buy me out completely, which she can only afford to do by topping up mortgage with the bank rather than having funds herself to buy me out.

The value of the property has increased since buying and also we've paid off 5 years of the mortgage so there is equity in the property that she will buy me out of and she can afford a mortgage of the difference on her salary at the moment. However, when we spoke to the bank mortgage advisor today he had no clue how it worked if she was buying me out, whether it was remortgage or top up or whole new mortgage for her. He is looking into and should have some better idea on it for us next week...but it wasn't exactly an encourgaing meeting tbh. We are keen to move ahead with it as soon as we can as she's looking to move jobs soon enough, so obviously would like to get it sorted while she's still permanent in her current job!

Has anyone any experience as to how it might work with bank in this situation? We have legal agreement in place for our share of apt with solicitors in place, so that should be straightforward once we get clarity on the bank piece!