Our aunt passed away without any will. Her estate has been settled and her traditional IRA is ready to be distributed to my sister and me in equal amounts. My sister would like to receive her share annually over three years. I prefer receiving my share in one lump sum.
Can this distribution be accomplished or does the distribution amount have to be equal each time they occur?

You can each receive your share in whatever fashion you choose.

It is up to the estate Attorney to make the necessary arrangements.