While your sister might not like the pre-packed supermarket products, the stuff that they sell on their deli counters is often far better. Unfortunately though, neither of the two big UK supermarket chains that have retained their deli counters (Waitrose and Morrisons) have stores in Northern Ireland.
Try Mike's Fancy Cheese Shop on Little Donegall Street
https://mfcheese.com/
Arcadia Deli on Lisburn Road
https://arcadiadeli.co.uk/
Sawer's at the Fountain Centre
https://www.sawersbelfast.com/
or Indie Füde on Ormeau Road
https://www.indiefude.com/