You, as co owners of the property, share equal rights and equal responsibilities.  If one of the owners is doing something against the interest of the other three or without their consent then it is most likely not a valid lease.  The one sibling would have to have the authority to bind the other 3 and it does not seem like that is the case from the way that you have phrased the question.  So I think that you should seek some legal help here with all of this.  All your names should be on the lease agreement if that is what you wish.  Now, I am assuming that the property is held outright by all of you and not in probate at this time and that the one sibling signing the lease is not eh personal representative of the estate. Then a different outcome could be had.  Good luck.