I can't give you legal advice, you should talk with a real estate attorney. Again, if it's in SF, contact the rental board, they will give you free advice.
Maybe you can clarify your question so myself and others can help you better.
Are you asking if they can claim ownership? Adverse Possession requires using land that you don't have a right to use and doing so in an "open and hostile" way for at least 7 to 10 years. The exact nature and application of this law will vary in different states, but you can start by researching this term.
Ultimately, you will need to work with an attorney to see if your family can make any claims, but remember, once you open this can of worms with the owners, they will be forced to fight to keep what they believe is theirs. It may not be pretty.