Dah - yes you signed two listing agreements, why didn't you ask the question before signing, why didn't you tell the second agent that you signed with another agent. They should automatically co-broke with each other, unless they are greedy.
Of course both siblings must join in the selling, you should both sign the listing agreements. othewise it will not be a smooth sale. Did both of you sign the two lisings?
However, It is very possible that your attorney can get you out of this since it still needs to go through probate. or just don't sell while its still in effect let them expire, how long did you sign for?
Good luck and if you have any other questions please feel free to contact me directly I will do my
best to point you in the right direction.