He died 6 years ago and now one sister lives there without benefit of a deed.

I cannot figure out how you could possibly expect someone to give you a meaningful answer unless you tell us what state or country you are in.

But, chances are, if he was unmarried when he died, after his estate is probated, all 5 siblings would be owners in common.

Was dad remarried ? So 6 years ago when he did, did the estate go info probate court to establish ownership,

Was dad the only person on the deed ?

Where is the property locateted at.