So long as the parents are married, your son (the biological father) has every right to have whomever he deems appropriate babysit the child. If your son decides to bring his daughter to you to babysit, he may absolutely do that.
At the same time, the mother can certainly be opposed to you providing any babysitting for the child. At that point, it is really an issue between the husband and wife. While, legally, she doesn't have to agree, I think we both know that your son may have a hard time "standing his ground" and bringing the child to you.
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