It is certainly possible for a witness to a birth record to have the same surname
as the child's mother, and to be related.
The function of the witness on these 19th century documents is to confirm that the
scribe has accurately written what the informant (in the case of a birth, usually
the father) has told him. If the grandfather came with the father, he would be a
convenient witness. If less than the required number of persons came with the
informant, the supplemental witnesses would likely be neighbors of the record-maker
who could be quickly rounded up to witness the fact that the record-maker had
accurately set down the oral testimony of the informant.
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