Can we get a separate contract stating we will not be responsible for the lease after cosigning should one party die or they get divorced?

You can make pretty much whatever separate contract you want with your daughter and son-in-law, including that you will not be responsible if they divorce or one should die, but no such agreement will bind the apartment owner.  The owner is agreeing to sell or lease (it's not clear from your question) only upon condition that you co-sign for the financing.  You can agree to different terms with your daughter and son-in-law that will give you the right to go after them if the owner sues you, but you can't compromise the owner's rights by an agreement to which he/she/it is not a party.