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Axel died because he exhausted his energy wiping out all the Dusks that attacked him and Sora. Having no Heart, the Darkness proved too strong for him and he faded away.
Roxas and Ven are not related by blood if only because Roxas is a confirmed Nobody.
Whichever relation they have is of an unknown nature at this time.
Axel died because he used the last of his strength to defeat the large amount of Nobodies in Betwixt and Between. And the connection between Roxas and Ven is currently unknown at this point, but it's unlikely they're related in a way such as that.
1. Agreed with Rain.
2. Roxas and Ven seem to be counterparts of each other and are related in some way. We will only know when Days and BbS comes out. Please refer to the 358/2 Days and Birth By Sleep Stickies for any more help.
Axel faded away because he got pushed to the max in that fight. I'm also guessing he lost the will to keep going because he lost all hope in finding Roxas.
Ven and Roxas couldn't be twins. Ven existed before Roxas was created.