Me and my sister bought a house can we still file separately?

You cannot file a joint return with a sibling.  You can only file a joint return with your spouse.  

If you bought a house together and are splitting the payments 50/50 and you choose to itemize your return, then you can each claim half of the interest on your individual returns. It is not required that you enter the interest expense, if you do not plan to itemize your return. 

Itemized expenses include mortgage interest, state and local taxes up to $10,000, medical expenses in excess of 7.5% of your AGI and casualty and losses in excess of 10% of you AGI with the first $100 not counting towards the loss.  Your health insurance and all medical expenses are only deductible for the amount that is over 7.5% of your AGI.  This means if your AGI is $50,000, then the amount that is over $3,750 is deductible.  

Then your total itemized expenses would need to be greater than your standard deduction below in order to benefit from your mortgage interest. 

The 2023 Standard Deductions are as follows:

• Married Filing Joint (MFJ)              $27,700
• Married Filing Separate (MFS)      $13,850
• Head of Household (HOH)             $20,800 
• Single                                                     $13,850                                

Blind and MFJ or MFS add $1,500

Single or HOH if blind add $1,850

Q. Me and my sister bought a house can we still file separately?

A. Can and must file separately.

Your real question is: what do we do about the mortgage interest and property tax deductions? 

You pretty much have a choice. One can claim it all or you can split it. It's usually best if only one claims it, allowing the other to use the standard deduction.

You have to meet the rules, which are:

1. You are legally obligated to pay it
2. You actually pay it. Paying from a joint account where you made sufficient deposits to cover the payments will usually meet this standard. However, paying from your own account would be a stronger audit defense.