Amerikanische Bürgschaft und Greencard?

"Green Card through family:

Many people get Green Cards (become permanent residents) through family members. You may be eligible to get a Green Card as:

an immediate relative of a U.S. citizen, this includes spouses, unmarried children under the age of 21, and parents of U.S. citizen petitioners 21 or older

a family member of a U.S. citizen fitting into a preference category, this includes unmarried sons or daughters over the age of 21, married children of any age, and brothers and sisters of U.S. citizen petitioners 21 or older

a family member of a green card holder, this includes spouses and unmarried children of the sponsoring green card holder
a member of a special category, this can include battered spouse or child (VAWA), a K nonimmigrant, a person born to a foreign diplomat in the United States, a V nonimmigrant or a widow(er) of a U.S. Citizen"

Quelle:

https://www.uscis.gov/green-card/green-card-through-family

Nichten und Neffen gehören nicht zum berechtigten Personenkreis.

Das könnte Deine Tante vor Ort weitaus besser klären, als Du von hier aus.

und bin ich dann zum teil auch Amerikaner weil meine Tante ja dort lebt 

Nein.

Den Rest kann dir am besten deine Tante beantworten.