Sorry this answer is a bit vague (too lazy/tired to research sources right now), but I believe there is some evidence that Crohn’s and MS may have certain biological mechanisms in common, and also that some treatments for Crohn’s also show promise for MS.
I have both Crohn’s and MS in the family, but quite distant relatives in each case, and not both on the same side of the family.
The MS is on my father’s side (one of his aunts), but the Crohn’s on my mother’s (her first cousin died of it - which is rare today, and I think was quite rare even then, but he must have been very unlucky).
Although MS is not regarded as hereditary, I’d always assumed a genetic disposing factor came from my father’s side, but knowing about the Crohn’s in my mother’s family, I wonder now if I got a raw deal from both sides.
There is familial clustering with certain other diseases, too, including Type 1 Diabetes, and Hodgkin’s lymphoma. It’s postulated that all these diseases share certain underlying auto-immune features in common, but the exact link is not yet understood.