I have to agree with oldtimer nothing good will come from separating them, they will grow up as cousins instead of sister and brother.
As your ex seem so hell bent on taking your kids away from you I would not be surprised if she secretly pushing her husband to get a job transfer to another state so they can move.
Its just evil what she is doing to your kids and damaging heard many stories from kids where the outcome is not good for the parents like your wife when the kids are older and looking back.....but there are some parents like you that lost contact with the kids because the ex poisoned the well.
Now is the time for them to set up healthy boundaries with the help from with a therapist and not just take the easy way (avoiding the wrath of their mother) and prevent FOO issues with future relationships.
