Ex Brother In Law Requiring My Sister To Pay Child Support

My sister have the custody of her son ( always had since her divorce 10 years ago), but recently her son asked her to allow him to live with his father which she agree with.They drafted a agreement with the help of her lawyer to let the son move from Florida to Maryland to live with the father for one year.The child is 15 years old now.The agreement has just expired and the father wants to renew it but is asking to get paid child support based on her salary and her new husband salary combined, is it legal?

Is my sister new husband obligated to pay child support for my sister´s son? Her former husband never asked for child support for the first year of the agreement because he owed a lot money in child support , but now he believes that is is even and wants money because since them he got a girl pregnant and needs to pay child support for the other child.He wants to get her money to pay the other child support.

What should she do? She is thinking about not renewing the agreement and have the child be sent back to Florida but at the same time is happy with living without the hassles of having to deal with him on a daily bases while she had the child with her. Another detail is that her new husband agree to put my sister´s son on his health insurance in with he pays for. Even with the treat of removing the child from the health plan if required to pay too much child support he did not back down. What should we do?