I think your question is seeking to determine whether or not this language will give them custody. Although it will be important in the court's evaluation of custody upon your passing, it will not be determinant of what the court will do with custody. The other parent will have the primary, constitutional right to seek custody of your son. The court's role in this circumstance will be to determine what is in the best interest of the child. The court will look at the language in your will to see that your intention, as custodial parent, was for your brother and sister-in-law to have custody. The court will determine that, as primary caregiver,  your intentions will be significant. The court will have to decide, however, what it ultimately best for the child.