Ummm, no.......I can just add a clause to my will that allocates parental responsibility and care to one of my family members in the event that my partner and I both pass.
Wills are about property. Your children are not property.
Further, decisions about where a child lives are made with regard principally to the interests of the child.
What a deceased parent might say in their will about where their child(ren) live(s)
after the parent's death is, at law, no more than an indicator of the deceased's preference.
Something said in a will about custody* is in no way binding on the executor,
nor indeed on the person named as preferred carer.
Even if the court does award residence to the named person,
that's because, in all the circumstances of that particular case,
doing so is in the best interests of the child, and
not just because that person was the one named in the will.
--------------------------------------------------------------------------
* Please excuse the old fashioned word, but it's the one most people recognise