I think there are many more similarities between IBS/IBD than the current understanding of disease process. People IBS/IBD (especially mild IBD) have more in common with each other than folks with other chronic illnesses. For instance, your sibling comments story about seeking treatment for IBS is very similar to many IBD stories as well.

IBD is itself a lumping of disparate diseases based on very minimal understanding of disease process, so I'm not sure what your point is.