Thank you for your input.
Regarding my Elder Brother in law I would not have said he is a reasonable person. He is an elder but some of his life choices are debatable and my sister has been belittled and mentally crushed by him.He has also been physically abusive and although the elders were made aware of his faults he was still appointed. He is extremely self righteous and if they were not both witnesses it is doubtful whether their marriage would have lasted. However whilst I can predict with a degree of accuracy what they may do but until push comes to shove I have no way of knowing what they will do.
Elders are human beings and whilst I would not vilify them all having known lovely men who truly slaved for the congregation I also have known a fair few who were poor examples. Some elders would certainly use DJeggnoggs approach but not this one.
I think in my families case the elder role is significant since my sister feels it gives them the right to police the family and criticise the rest of us. It also means they are precious of their status and do not want to do anything that may be seen to compromise their position.
Thank you all for all your responses I think I am seeing a way to approach this. I think sometimes you have to accept that you can't achieve a perfect solution only the best one in the circumstances.