Hey, thank you everyone for your replies, sorry that it took time for me but I will give a further explanation about the case.
My grandmother is 85±, she is a US citizen who lived in the US for maybe 1-2 years (in total before and after she got her citizenship). She filed tax returns in the past and she has an address in the US. My father (her son) filed an application (F3) and the current date is getting close by few months. We can assure that if we get it she will also move to the US with us.