Cat stopped using all litter boxes after new kitten

We got a new kitten Suzie (1F) 6 months ago, and our adult cat Nell (3F) had a tough time adjusting. At first, we kept them on separate floors in our home, until Nell was comfortable being on the same floor. Now Nell will sleep in the same room but no physical contact.

BUT, we have multiple litter boxes, and Nell has stopped using them since we got Suzie. She demands to be let out into our yard and does her business there. If she is not let out, she will use corners and hidden places instead. There is no way for Nell to have a private box as suzie will end up using it at some stage. We never had any litter box issues before we got a new cat. Any advice or tips would be highly appreciated!