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.
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.
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