You should check with a lawyer licensed to practice in Pennsylvania. Generally, as an "heir" you and your sisters would be entitled to notice of any proceedings. You could call or visit the probate court in the county where your aunt lived to ask if a probate has been started and request copies of the file.
DISCLAIMER: This answer does not begin an attorney-client relationship and does not constitute legal advice.
An allegation of undue influence can negate the validity of a will. Assuming that probate has not yet begun, you may challenge the granting of letters of administration. If you are an heir to the will, you will receive a copy of the request for the granting of the Letters. If you have not received such a document you should contact the Register of Wills to determine if the probate process has begun.
Contact an attorney experienced in the challenging wills. From experience, I can tell you that the process is often long, arduous, and can be expensive for both sides. However, it may be well worth it. I you have further questions, please feel free to contact me.