Thomas, 52 - who is half-brother to the new Duchess of Sussex - is reportedly facing a drink-driving charge after the arrest in Oregon.
Officers said he was visibly intoxicated when he was pulled over in the early hours of Friday in Josephine County.
Mr Markle Jr's sister, Samantha, also added: "His drinking is not related to Meghan".
Royal fans were shocked by the breaking news, with one commenting: "And yet her family members wonder why she doesn't want them around!" They are expecting their first child.More news: Video shows bus driver rescuing baby wandering alone near road
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Per authorities, Thomas had a blood alcohol content of.11% and was issued a citation for a DUI. A long-serving aide to Queen Elizabeth, Samantha Cohen, was tasked with giving the former actress "duchess lessons" for six months after her wedding and left the role in November, as planned, after 17 years of service. A second moaned: "Put his name in the headline and leave hers out of it, she has nothing to do with it".
"Absolutely I think our wedding will bring the family closer together", he explained.
Meghan, The Duchess of Sussex, arrives to visit the Royal Variety Charity's residential nursing and care home Brinsworth House, in west London, Britain, December 18, 2018.