As far as Dhundubi is concerned, in Valmiki there is just a passing mention of "the demon Maya, elder than Dhundhubi." When describing Dhundhubi's fight with Valli there is no mention of this at all. It is very frustrating. I wish I knew Sanskrit well then it might be easier to figure it out. Now I am just placing my trust on the English translations and there is no clue as to the competence of the translator.
Dhundubi is a very powerful rakshas who is itching for a fight with anyone and everyone. Dhundubi goes to the ocean and challenges him to a fight. The ocean refuses and wisely transfers Dhundubi's attention to Himavan. Himavan refuses to fight with Dhundubi and tells him to go fight with Vali as Vali is the better match in valor for Dhundubi.
In this incident there is no mention of Mayavi or whether Dhubdubi is a brother of Mayavi by Valmiki. Valmiki just writes that Dhubdubi is full of arrogance because he has the strength of 1000 elephants. Therefore he goes around challenging all and sundry for a fight. Also, Dhundhubi is in the form of a buffalo.
As far as Tara and Rumaa being sisters, there is no mention of that. Actually Rumaa's name is mentioned only 2-3 times I think. Also, Sugriva mentions to Rama that Vali has taken his (Sugriva's) wife, that is all. There is no information on Rumaa at all. Her name is mentioned as Rumaa and not Roma. Later when Lakshman goes to Kishkinda to warn Sugriva for forgetting his duty to Rama, Rumaa is mentioned as standing next to Sugriva.
I hope this helps Vibs and Vrisha.