Once the person has died, all heirs are entitled to receive a copy of the will. Before then the will is available to only the people the testator (the person making the will) wants to show it to. If your sister has a copy of the will she is legally not allowed to share it with you unless the testator wants her to do so.
As an heir of the estate you are entitled to a copy of the will and notice of any probate proceedings.
Any individual seeking legal advice for their own situation should retain their own legal counsel as this response provides information that is general in nature and not specific to any person's unique situation. Circular 230 Disclaimer - Advice given in this response cannot be used to eliminate penalties with the IRS or any other governmental agency.
If the person has passed away, the holder of the Last Will and Testament must file it with the Register of Wills (usually in the county where the decedent last resided). Then it becomes a public document. So, you could go up to the Register of Wills office and request to see it. Also, the Register of Wills would send you "Notice" as an "interested person" that the Will was filed. If the person has not yet passed away, then you don't have a right to obtain a copy, in fact the executor may not necessarily have a copy. You may wish to speak with an attorney to futher clarify your particular situation because this is general information.