Step one is very simple. You get a lawyer. Two estates have to be probated - the intestate one for your father and the testate one for your aunt. Additionally, depending on whether your dad and aunt ever completed needed paperwork on your grandmother's will, someone may have to be appointed in THAT estate to complete that work, Your cousin's death may add a fourth estate to probate.
For an experienced probate lawyer none of this, absent a large estate or arguing heirs is a "wingdinger" but this is far too complex for a pro se litigant as the probates of several estates need to be coordinated.
Mr. Ashman gives you good advice. The issues are not substantively difficult, but you can certainly expect it to be complicated procedurally, and you should understand that any minor error made with regard to any of the estates will likely have repercussions with regard to the other estates.
You need to promptly hire a local attorney.
Good luck to you.
Michael S. Haber is a New York attorney. As such, his responses to posted inquiries, such as the one above, are limited to his understanding of law in the jurisdiction in which he practices and not to any other jurisdiction. In addition, no response to any posted inquiry should be deemed to constitute legal advice, nor to constitute the existence of an attorney/client or other contractual or fiduciary relationship, inasmuch as rendering legal advice involves the ability of the attorney to ask appropriate questions of the person seeking such advice and to thus gather appropriate information. In addition, an attorney/client relationship is formed only by specific agreement. The purpose of this answer is to provide the questioner with general information, not to outline specific legal rights and remedies.