Child's mother (my sister) is terminally ill. The father of the child works overseas. My husband and I are preparing for a moment when we will have to step in and take the child to live with us. I would like to sponsor my sister (and hence her child and husband) for a green card, but I am not sure if I should file now, or wait and apply just for the child.
Personal Injury Lawyer
You need the advice of an immigration lawyer in the US to answer the immigration aspects of this question. As for guardianship, absent the natural father's consent, you are not likely to get that unless the child is already here in the US and the father is unavailable and not opposing it; otherwise, that is a matter for Polish law. Temporary guardianship may be granted if the child is here and there is no parent here available or willing to take the child.
Immigration - coupled with guardianship - is complicated. Please consult an immigration attorney. There may be steps that you will have to take before applying for a green card and an immigration attorney will need all of the facts before he or she can advise you.