Deeds, like all other estate planning documents can be contested on a number of grounds, the two most common being lack of capacity and undue influence. There are other potential issues, as well, such as delivery, depending on the facts of the case. There is not really enough information in your summary to determine what the issues or concerns are, in your situation. Please feel free to post additional information, if you have other questions.
This is a good question because you describe a common situation. I would need to know more about your sisters' and their father's goals. It sounds like one sister is taking care of her father. Many times this means she is doing A LOT of work, putting in tireless hours, and sacrificing her own needs to care for their father. If this situation is like many that I see, then it seems reasonable that the family would want to create the opportunity for your sister to get the deed, or a share of it, when the father dies. (Of course, it could be the opposite: she could be free-loading, trying to take advantage of their father, and attempting to get a deed that she doesn't deserve!)
Any document can be contested. But not everyone who contests a document wins.
In this situation, it sounds like the family should meet with an elder law attorney and discuss what the ideal desired situation is. Hopefully, everyone agrees who should end up with the deed upon the father's death. An attorney can recommend the "right" legal documents to use: living trust, life estate deed, lady bird deed, quit claim deed, joint life estates with cross-contingent remainders, will, power of attorney, scribbled scrap of paper, etc. Each technique has advantages and disadvantages and the result will depend on how they are used.
Even if a family meeting isn't practical, or there is no consensus, the correct documents can be put in place by the deed-owner to provide for clear results the increase the chances of everything avoiding probate court and avoiding a family fight, while minimizing any tax consequences.
If the deed is the father's and if there is a chance he may need long-term care, or if he is a veteran, then setting it up the right way may also make it easier for him to get Medicaid benefits or veterans benefits. For Medicaid info, see http://www.mymedicaid.com.
Hope this helps! Sounds like the sisters are lucky to have someone like you helping to find out their options!