Something is, well, very fishy here. Your brother-in-law "needed to file for temporary joint custody so that he could travel with the child"?? Huh? That's simply not true. Did you give him custody? If you did, then, it could indeed be a problem if you just took the child back.
In sum, "it doesn't make sense to me", either. You might want to check this counsel - don't have a referral handy for Essex, but look around on Avvo.Ask a similar question
Your question is a bit unclear as to whether there is any litigation now pending. That question needs to be answered beforehand. But, if you are seeking to leave on a vacation, there is no need to file for custody.
DISCLAIMER This answer is provided for educational purposes only. By using or participating in this site you agree and understand that there is no attorney client privilege between you and the attorney responding. This site cannot be used as a substitute for competent legal advice from a licensed attorney that practices law in the State where this offense is charged; and, who has experience in the area of law you are asking questions about and with whom you would have an attorney client relationship. The law changes frequently and varies from jurisdiction to jurisdiction. The information and materials provided are general in nature, and may not apply to a specific factual or legal circumstance described in the question, or in the State where this charge is filed.Ask a similar question
Sign up to receive a 5-part series of useful information and advice about child custody law.