Richards treatment of the Countess of Oxford

Dear All - I have read somewhere sometime ago & I cant remember where, that Richard
treated the Countess of Oxford very badly - something to do with him getting
possession of her land/properties etc., Since then I have been unable to find anything on
this matter - it is not mentioned in Paul Murray Kendalls book or any of the other books I
own. If I recall correctly it was implied that he had stressed her out so badly it contributed
to her death. Now I am not believing it actually happened this way but I would appreciate
enlightenment on what actually did take place. I presume she was elderly - also am I
correct in thinking she was Warwick's sister.
best wishes Eileen