My sister now has dementia. How can I remove her name from our property house and land deeds?

Short answer: you can't.

Why do you figure you need to?

If she is indeed mentally incapacitated, and if she never wrote a will, when she dies the property will go to whoever the law in your jurisdiction provides will inherit under the intestacy ("no will") statutes (unless the state takes her share to reimburse whatever has been paid for Medicare costs).

Were you or someone given power of attorney for her? Or did a court appoint someone to oversee her affairs? Unless that happens, she iis not capable of signing any legal forms.

As AK noted, why is this an important issue?

Although with dementia and not able to speak for herself, she is still part of the family unit and should be thought of as such and respected.

You can't just take someone's property because they are incapacitated. If you are her heir, it will go to you when she passes away. If you are not, you will have to wait until she passes and try to buy her share from her heirs, and refinance to get them off the paperwork or, if her heirs have power if attorney you may be able to buy out her share now if they are amenable .

The court can appoint an attorney or other person to act on their behalf. But I doubt if anyone would view taking their ownership of property away, as a benefit. So most likley you would have to pay them for their share of the property.