But then what wand did Voldemort use to duel James and Lily and to cast Avada Kedavra?

It's not really clear whether Bella knows how Voldemort died. She recognizes Harry from his scar, and it's improbable that she could have been captured so soon after Voldemort's death that she didn't have time to read the newspapers. But she also says:

"My... Lord... I went where you said to await you, but you did not come... I looked for you but I could not find you..."

Which would be an odd thing to say if she knew his body had been found on the floor of Potter's house. Maybe she knew about the horcruxes and other dark magic, and figured Voldemort losing his body would be only an inconvenience?

So either Voldy gave Bella his wand and used a different wand to duel the Potters (which meant he knew something was up and probably planned to die), or she just took the wand from the corpse or stole it from the authorities or something.