Witness at a Wedding — the Brother of the Bride’s Sister-in-Law

At the wedding of a secular couple, there are not enough Shabbat-observant witnesses. Can the rabbi serve as one witness, and can the second witness be a religious person who is related to the bride: the brother of the bride’s sister-in-law, meaning the brother of the wife of the bride’s brother (the son of the in-law)?

Greetings and blessings 

The custom is to avoid this. 

But if there is no other possibility, it is permitted. 

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