Is it legal to leave a child to someone via a will?

No. It is not legal to bequeath a human being via a will.

And, contrary to popular belief you can not transfer, appoint or otherwise give guardianship of your minor children to another person - family member or not.

Once legal parents or legal guardians are out of the picture a new guardian must be appointed by a judge in a court of law.

One can make their wishes known through various methods as to whom they’d like/prefer to assume guardianship and/or custodial care of their minor children - but ultimately it is up to a judge.

Your sister was wrong. Even if this was the wish of your Mom, even if mentioned in a will, you still need to go thru the courts for guardianship.

My sister lived in MD. She had a minor son. My brother in NC. Brother needed to establish guardianship in the state of NC where he lived. The court allowed the cost of guardianship to come from my sisters Life insurance policy where my nephew was beneficiary. My other brother and I had to sign paperwork saying we would not contest.

I would think Mom had custody thru the court plus guardianship. You need to contact Family Court and tell them sister not only took ur kids but she took them out of state. Which even with custody/guardianship I would think is not allowed if she is planning to live in another state.

Good Luck and come back and tell us how things are going.

Children can’t be willed to someone else. Wishes can expressed but the will isn’t a legal document when it comes to child custody.

To do this is a bit risky, in some cases it can be a while after death that a Will is read. Guardianship should be established and be separate from a Will.
Technically, and there are no specifics in your post or profile your sister could be charged with Kidnapping if she has no legal Guardianship of your children.
If your question is how do you regain custody of your children this is something that you need to discuss with a lawyer and the sooner the better.

Prior to a long trip overseas 20 years ago, my husband and I made out our first wills. We had 3 small children at the time. The will included naming our desired legal guardians for our children if we both died. But, even if the will is executed properly I believe there is still a legal process to finalize the guardianship through the courts. Having custody and a finalized adoption are 2 very different things and the adoption was interrupted by her death. Custody may not be the same thing as guardianship. Every state may not have the same laws and process so you must consult an attorney for accurate answers as to your options.

Yes, wills often mention care of the children. Who is to care for them in the event of a death. But, check with an attorney especially in the case of temporary guardianship. Child protective services must be involved somehow. Check with them first.