Thank you for this.
I can see that this is a quite complicated issue as it seems to involve significant issues of trust among other things. The reality is that your son had deceived you. We know little though about the motives of this deception and this whole behavior. There are many questions that you could ask yourself. Could he have asked you for this money?and what would have been your reaction?Did
he actually need this money or did he use it for recreational purposes?Has he done anything similar in the past or is this the first time he is breaking your trust?When you say that he said that it was his right to take the money what do you think he meant? Could this statement have meant that he was feeling very defensive when you spoke to him and he saw the reactions of his siblings?Knowing your son, do you think that he understands and realizes that he has broken your trust or do you feel that he usually uses people and disregards XXXXX XXXXX?These answers would help you see the problem from many different perspectives before you decide to take any action.
At this point, I would advise you not to listen to others but to try and resolve this yourself. Also, in order to avoid a split I definitely feel that you should not involve his siblings any more in this and just tell them that this is between you and him and you will handle it yourself. If you involve them more than they already are, then I am afraid that your son would not be able to re establish the relationship with any of you and there is going to be a split in the family.
Now, in terms of your relationship with your son, what will happen depends also on what you would like to happen. If you feel for example, that this is the first big mistake he has done then you may want to consider all the above questions and whether you would be willing to put this behind after you had a good and serious chat about the incident. In any case, you would need to clear the air between you. He needs to know how this has affected you and you may need to understand what made him go behind your back. The conversation should have no judgmental character but the intention of understanding the circumstances around this behavior. So it should ideally be a calm conversation between just you and him without putting blame but at the same time expressing your feelings and thoughts. If your relationship has been a strong and solid one, then it should survive this blow as well. We all make mistakes, as long as we can accept them, rectify them if we can and ask for forgiveness. You could also ask him what he thinks that he could or should do to fix this broken trust. You would need to give him the responsibility of accepting his actions and amending them if possible. If you do need the money you could possibly agree on a negotiation. If you do not, he should still need to work on you trusting him again in some ways.
On the other hand, if he has done similar things in the past and so you feel that you can no longer trust him ever again, then this is understandably a different issue and you should have different expectations of him, if any. Relationships are built on trust and mutual respect and if this is not there, then the relationships become pretentious. The split in this case would be inevitable.
Please take your time to read these thoughts and process them in your mind. Feel free to share any feedback on these thoughts.
All the best