Sounds like you will only be able to carry your nephews as dependants. You wont be able to claim EIC if you cannot prove they lived with you 6 months or more of 2019. So to file as dependant you can ask your my to write out a note saying she gave you permission to file the kids(that letter needs to be notarized with a copy of her ID on it, dont forget to sigh and date) she will also need to give you those custody papers ( so you can get a copy made) to prove she has custody. If you are claiming EIC. Whose address did you use to file? If you used your moms, does the kids school have on file that they lived at the same as your mom If so you can claim head of house hold and claim EIC. If you
