My sister lives in Alabama her husband just pasted away now his parents took her kids and wont let her have them back only one is blood can they do that?

They probably don't have "the right" to do that, but whether they can or not depends on whether you and your sister can mount an effective legal challenge to win them back, if simple persuasion doesn't do the trick.

And that might depend on more than black-letter law. For example, if there are skeletons in your sister's (or maybe even your) closet, then those will be marshaled against her by her in-laws' attorneys. What's your sister's history with drugs and with other men, for example? (These are rhetorical questions: I don't want you to answer them here. Talk to your attorney.) What were the circumstances of your brother-in-law's demise? Is there ANY uncertainty as to parentage, marital or adoption status?

There are a lot of issues that strangers on the Internet can only guess and wonder at. But the facts are in Alabama, and you need to find an attorney who can work with them. Or around them, if need be.

NO they can't unless they have some legal right to do so, if not then it can be considered kidnapping.

Many questions would have to be answered to give you an answer. Talk to an Alabama family law attorney if you want to have your children live with you.

She has parental rights for the child who is a blood relative, and the parents cannot take the child legally without a court order. For the other children, she would have parental rights if she legally adopted them, but not otherwise.

She should see an attorney, in any case, as the practical consequences of these laws can be complex and vary by state.