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How old are the children?
Did they both express they want no contact with the sister and aunt as well as the father?
I am glad to hear that you are doing the right thing even though he abandoned you and your children. It sounds like your children are confident in their choices and at their age they should have have those choices upheld (Provided there are no court orders that give the aunt or sister any rights)
I would get in contact with the aunt and the sister and explain the situation to them. Let them know that per your children's request they are not ready yet and when they are you will be more then happy to put them in the right direction. Ask them for updated information on how to contact them, and request that they update it as it changes in the future so they know you are serious about forwarding it to the children once they are ready.
Hopefully the aunt and sister will respect the children's wishes.