Sad news. I remember watching Marvin Webster back in the '70s/early '80s. Condolences to Martell and the rest of the Webster fam.
Update: Posters have questioned whether Marvin Webster was actually Martell Webster's uncle. The original version of this story stated that this was the case, but it was redacted from later versions. I have long seen the Marvin/Martell connection stated as fact on the Internet. It's possible that this is just not true and that Marvin's death has led to some factchecking.