Sure, there is still the matter of the mortgage company. A transfer w/o thi provable can sometimes result in a default, so clear the path and get a real Tate lawyer to transat the transfer. There is much more to it than the preparation of a deed form.
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The quick answer is yes, very simple for him to sign over a warranty deed to you and your sister so you both own it. You may want to chat with a real estate attorney to sort out HOW you will own that property together (e.g. Joint Tenants with Survivorship or perhaps Tenants in Common, where you'd effectively each own 'half').
The 'Lawyer answer' is to remind you that this property has a mortgage on it; the mortgage most likely has a 'Due on Sale' clause which more or less means that they could require the loan be paid off/paid in full because the ownership changed. The lender may not care, but again that's how the Security Deed will most likely read. They may not care as long as payments are coming in but be aware that they 'could' call the loan.
So yes, he can easily transfer the property to you, but note there are potential issues.
Just plain over-the-counter commentary, no lawyer-client relationship created. Seek your own counsel for true 'legal' advice. This is just an off-the-cuff response to a short question on the web; not to be construed as legal advice.