As a general rule, I advise against people attempting to borrow more than the banks are prepared to lend, unless they have very good reason to believe that their salary is definitely going to increase substantially in the short term. Interest rates are very low at present - they could rise substantially again during the life of your mortgage. How would you cope? If a bank thinks you cannot afford a loan, why do you believe otherwise? Would you not consider waiting until your income has increased or something comes up that is within your price range?
But to answer your direct question, if there is only a small gap between what you want and what you qualify for on your own, your sister could be guarantor in which case she is not a co-owner on the deeds of the house. If the gap is large, lenders will generally want her named as a joint party on the mortgage and possibly the deeds, i.e. to be a joint owner.
In any event, your sister must not only have the income, but she must not be too heavily encumbered with debt herself. This arrangement will also impact on what she can qualify for, if she wants to borrow for herself later.