Please tell me the Siblings can take a golf cart to the Church, that is so far! I would need one! Lol.
I think the abbey HAS to be accesible even if it's all old buildings. The paths connecting everything were probably pretty cobble but have changed to smooth brick so siblings and ghouls with mobility aids can move freely.
There's probably lots of wheelchair platform lifts lining the stairs of buildings and even outside.
Still I think there would probably actually be golf carts. Maybe the longer ones that have like 3 benches. It would be like a little school bus to take siblings and ghouls to and from scheduled events like mass or celebrations.
But if everything else fails, be sure that a big strong ghoul will help you get around if needed.
(Wouldn't it be fun? Like "Oh no! Im stuck late at the library on my own and its raining and I've got to get all the way to the clergy quarters. I'll get soaked and my wheelchair is going to get all muddy. Whoever will come to my rescue" so you press a button next to the door to call whoever ghoul is on mobility duty, so they can help you get to where you need to be, or at least hold your umbrella and walk with you?)