Complex sentence: They are needed in order to demonstrate that an idea is sufficiently to merit a dedicated article about it; and for a fringe theory to be discussed in an article about a mainstream idea, reliable sources must discuss the relationship of the two as a serious matter.
Split into simpler sentences: They are needed in order to demonstrate that an idea is sufficiently to merit a dedicated article about it. For a fringe view to be discussed in an article about a mainstream idea, reliable sources must discuss the relationship of the two as a serious matter.