Jada,
I am sorry that you have found yourself in a uncomfortable position but I think you are doing the right thing by informing his mother what has been happening. I do feel that this will work out for the best. His mother maybe alittle upset/shocked at first but hopefully she realizes that he has been in contact with his sister for some time now and is handling it well. Sometimes it is so hard to take the first step but once it is done for you, you find that you really did not have anything to fear at all!