I see where you are going with your logic and I like it. I have have a few questions:
Who is Sarah and where is she listed as Matthias' sister?
When was it determined that Nicholas and Elizabeth were kin?
When was it established that Sarah was sister to either or both Nicholas and Elizabeth thereby making Matthias a brother to them all?
I think that it has been established that Richard was related to Oliver (I believe that Elizabeth has already been established as his daughter but I may be incorrect) but I cannot remember if it was father,brother,cousin.
Proving Matthias to belong to any family (including that of Oliver) is extremely difficult. I do not think that we will ever be successful. I am a Project Administrator for the Sension Family DNA testing group at Family Tree DNA. What I have learned is that the science is absolute in matching lineages BUT there have been so many illegitimate children and adoptions that paper trails and DNA may not match up. For example, Sir Oliver was purportedly a Russell, and even though the paper trails say St. John his DNA may read as Russell. It grows even more complicated when you add the fact that the Russell's may have had an illegitimate child in their midst.