My brother left the country about 10 years ago and has PR status (RO not met). He wants to come for a vacation and not as a PR - no intention to work or live here. At the same time, he doesn't want to go through the hassle of renunciating his PR. He plans to come in by car from Seattle (Peace Arch). British passport. Will he likely have issues coming to Canada if he already has ESTA?
ESTA is a travel authorisation for the US, and it’s irrelevant for admission to Canada.
If he shows up at the border without a valid PR card, he will likely be referred to secondary inspection to establish his compliance with the RO. He might also get reported for it, which can lead to termination of status. That said, because he is still a PR he will be admitted into the country.
If he has no intention of living in Canada, I would suggest renouncing PR prior to travelling to Canada. This will reduce or eliminate any delays at the border, and it will allow him to apply for an ETA and board a plane to Canada directly, without the detour via the US.