Originally Posted by slamjet
Get your Bishop and Stake President involved.
slamjet is right. You need to speak with your bishop and stake president.
A child may be sealed to one natural parent and one step parent only when:
a) the natural parent has legal custody AND physical custody for the marjority of the time
b) the other natural parent has given signed, written consent naming the child to be sealed and the couple to whom the child is to be sealed. This letter must be presented at the temple.
In the absence of these conditions, children cannot be sealed to a step parent. It sounds to me that you haven't given written consent. Let your bishop know what you think is happening, and he and your stake president can contact your ex's priesthood leaders to ensure that policy is followed. If they feel that the concern is not being taken seriously, they can also contact the temple where they are planning to be sealed. Temple leadership will take these issues very seriously and will not perform the sealing without the proper documentation.