Can I use my Sister as a Cosigner for a Personal Loan. She has great credit.?

Yes, if your sister is willing to co-sign and she really does have a 780 then you guys should have no problems getting the loan. However there are 2 things to consider, first make sure her debt to income ratio isn't to high and if you're going to be primary on the loan make sure you have doented income to cover the payment of the loan.