The Father needs to file a Petition for Modification of Custody as soon as possible. He can file this Petition in the county where the ex spouse resides, or where the last Order was entered depending on the circumstances. In order to file such a Petition you must be able to prove that there has been a "substantial change in circumstances" since the entry of the Order granting the ex spouse sole custody. The fact that she has been charged with a felony, and will most likely have to do time will be sufficient for you to file your Modification of Custody action.
How long have the children been staying with the father? Where does the ex spouse reside, or where did she reside prior to being incarcerated?
The maternal grandparents will also likely have to be joined as a necessary party as it seems they are the current temporary legal custodians. I strongly suggest that you contact an attorney to discuss things further as soon as possible, as the father will not have any formal legal rights until a Court Order grants him such rights.
I would be happy to discuss things with you further in a private and confidential consultation at no charge to you, so that you can get a better understanding of the steps and costs involved to file a Petition for Modification of Custody. I can also use that time to get some more information from you, so that I can provide you with better advice.
Please know that this is a general interest posting and does not create an attorney-client relationship.
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