Hi C.,
Having gone through IVF with an egg donor, I can tell you it's a big committment to become a donor. The going rate is around $5 to 6k for the donor because it's a big time committment and harsh on your body. You start with birth control to synch your cycle with the recipient and then begin Lupron and other drugs to stimulate eggs. During the process you are going to the IVF clinic every other day or so so that the progress of eggs can be monitored through ultrasound and your meds adjusted, if necessary. Any IVF clinic can walk you through a typical cycle and the drugs involved.
I considered using my sister's eggs because I thought her's would be as genetically close to mine as was possible. I am glad I did not end up going that route though - my sister is not the most emotionally stable person, to say the least, and I never want to deal with explaining to my kids who exactly their egg donor is and having my kids know that person and also with whatever emotional attachment my sister would be bound to feel for my kids.
Dealing with infertility was difficult to say the least and I totally understand your Aunt's desire to keep the genes in the family so to speak, and it also could reduce costs, if you truly "donate" without compensation. BUT it creates a VERY complicated family dynamic even under the best of cirumstances. Be sure you can truly handle it emotionally.
Also, please also keep in mind that egg donation very often produces more embryos than are necessary for 1 cycle. I think you relinquish all rights to any of that material and if they chose to freeze any embryos, they can possibly have more children later, make sure you're on board for that possiblity too.
Please feel free to contact me if you have any other questions about the process.
Good luck deciding and do not feel bad at all if you decide not to, it's not selfish to do that.
Best,
S.