Deductions & credits

1. A shared account means together you are paying all the bills. Only you two know who puts in how much and how you figured the amounts going in and for which bills.  The IRS does not want you claiming something you are not entitled to claim. You are both entitled to claim mortgage interest if the house is in both names. You are not required to claim the mortgage interest and most people don't. 

2. That said, the IRS believes you should claim the part for which you pay.

The IRS doesn't have much to say which makes it harder. The only thing the IRS says is what to do if you don't get the interest form, as seen  on page 11 of About Publication 530, Tax Information for Homeowners is:

More than one borrower. If you and at least one other person (other than your spouse if you file a joint return) were liable for and paid interest on a mortgage that was for your home, and the other person received a Form 1098 showing the interest that was paid during the year, attach a statement to your paper return explaining this. Show how much of the interest each of you paid, and give the name and address of the person who received the form. Deduct your share of the interest on Schedule A (Form 1040), line 8b, and enter “See attached” to the right of that line.

3. Since she earns more, chances are she pays more or maybe all of the mortgage, depending on how you set up your finances.

However, most people take the standard deduction these days so the chances are high neither of you itemize but maybe one of you can. As a couple that has set up your finances, you can arrange them to pay bills as you please. The IRS has this chart of deductibility from Pub 936.

This would be a judgement call on your part for what you feel could be proven to the IRS, if they ask someday. Just for kicks, put it all in and see if it even matters. It may not matter and then you won't need to worry. If you both end up with the standard deduction, no need to decide anything.