Contact with (maybe?) Positive case - likelyhood of catching it?

[MakkaPakkas]

Hi, I can't find a good answer to this. I've been round my BIL's house today to visit the DN's and he has symptoms (fluey, cough, snotty) etc... He is not the type to test and I felt awkward to just leave so sat there on the other end of the sofa for about 40mins. I'm double vaccinated, but now panicking I'll get covid and ruin my son's first week back at school by spreading it around our family.
I've read that delta has an R0 of 6-7 in a naïve population, but not sure how that might apply to this situation. Anyone got any good data? Or done a similar thing and was ok?
I feel very stupid now for not just saying something and going home, but I'm not in the area often and wanted to check in on them.

After a trawl I've found this: www.google.com/amp/s/www.imperial.ac.uk/news/227713/coronavirus-infections-three-times-lower-double/amp/
In addition, double vaccinated people were less likely than unvaccinated people to test positive after coming into contact with someone who had COVID-19 (3.84% vs 7.23%).

Hi OP 2 members of my household have tested positive in last 2 1/2 weeks.
I have been in VERY close contact with 1 of them (my DP) & eaten dinner with my son every evening.
I’m double jabbed & I've been negative .
I’m not even sure DP caught it from my son because of the timings- we are sure he caught it somewhere else.
Our lodger who is unvaccinated has been negative as well.

I read a report yesterday in Daily Mail online that scientists found that fully vaccinated people are 82% less likely to be infected than the unvaccinated.