Do the former husband's family have the right to take the child from the widowed mother if she remarries? Can she marry after the child turns seven? Is the child given a choice?

If the suitor is not a relative of the son (such as his paternal uncle), then your marriage forfeits your right to custody, based on the Prophet's (peace be upon him) saying: "You have more right to him as long as you do not marry." If the son reaches seven years of age, the preponderant opinion is that he is given a choice between his parents. It is not possible to know what the outcome regarding custody will be upon your marriage, because the ruling depends on the judge's madhhab (school of thought) and the child's choice.