Does the timeline work out for Melessa Blackwood (mother of Brynden Rivers) and Melantha Blackwood (mother of Lord Edwyle Stark) being sisters?

Good question! This sort of stuff isn’t my strong suit, but it’s always a good idea to stretch one’s muscles. 

Sisters is a little unlikely. Brynden was born in 175 AC, so his mother would have been born 156-160 AC. By contrast, Melantha’s husband Willam died in 226 AC at the Battle of Long Lake, and it’s unlikely that he was 66 at the time. 

More likely, Melantha was Melissa’s niece.