An attorney should be able to clarify the situation and get the immigration detainer lifted after he/she has had the opportunity to assess the criminal history; and has determined that he is not removable.
Contact an experienced immgration lawyer asap.
Best of luck.
You should definitely get an attorney to review your brother in law's situation. Once his criminal sentence is completed, he will be transferred into ICE custody for removal/deportation proceedings. If there is potential relief from removal, an attorney could get him a bond and defend him in immigration court.