When? I was going by general info for the requirements at many embassies and consulates.
The UK embassy site states this.
"Proof of relationship to a family in Thailand, e.g. a copy of marriage certificate / birth certificate / certificate of adoption
Personal details of a family in Thailand, e.g. ID card, Passport and the visa page or stay permit in Thailand"
Source: https://london.thaiembassy.org/en/publicservice/84508-non-immigrant-visas?page=5d6636cd15e39c3bd00072dd&menu=5f4b6eb3f6ae4b236972c562
What if the family member did not have a Thai passport?
Best to be prepared for anything.
Things do change.
A little different now. https://thaiembdc.org/2020/07/11/foreigners_family/
Also much different it you were applying for it at a Honorary Consulate before covid 19.