Valmiki's Ramayana: the inexplicable

After reading the Ramayana everybody asks how Rama could kill Vali hiding from him and how he could renounce his loving wife Sita after taking all the effort to save her. While I am trying to figure out the answer to these questions (I doubt if anyone can convince me that Rama was correct), I realized I had a couple of more questions besides these:

1. Sugriva complained to Rama that his elder brother Vali had illegally taken Sugriva’s wife Ruma as his own. This was one of the main reasons Rama killed Vali (To correct the ‘adharma’ that Vali had done.) How then could Sugriva take Vali’s wife Tara as his own after his brother’s death? Isn’t that unrighteous too? After all, in Indian culture, your elder brother’s wife is equal to your mother…

2. How can one justify Ravana’s brother Vibhishana betraying his brother to join Rama in battle. Clearly he had the selfish gain of obtaining Lanka. When Rama is close to death after Indrajit’s attack, Vibhishana laments that he may never be able to rule over Lanka, rather than worrying about Rama’s health. On the other hand, in the Mahabharatha, Karna stays with the Kauravas even though he knows that they have wronged and will be on the losing side. Isn’t he a truer brother?