IRS does not disclose or release any information about tax payer to a third party unless a power of attorney being submitted or a Federal warrant issued by Judge.
However, you may do one of the followings:
Ask your sister to let you know or show you about her income tax return of certain year.
If she does not remember and she wants to let you know but she could not find it, you may download form 4506T and fill it, then ask your sister to sign it in person, then put your address as returned address.
As soon as the IRS process the form (request) you will get your sister transcript (upon her
Finally, no way for any one to ask the IRS by phone for Tax year transcript unless the IRS verify the following:
Full name, Date of Birth, Current Address, Filing status. Addition to this, IRS may ask for additional information such as: previous address, total income, number of dependents, full name of spouse and date of birth if married.