Just skimmed through the video, pardon my beginner Japanese. There wasn't any context on why they were at the beach. The 3 changed, and Rietta got intimate with Kurt. Arika and Terena saw, commented on Rietta's boldness, then Terea approached Kurt. Arika followed after, a bit jealous of her sister. Immediately after, Arika started riding kurt, he came outside (reason unspecified). Then Terena took over and said it's fine to finish inside her. Arika was a bit shocked, and mentioned to Terena that they promised to only allow nakadashi's after marriage, and that Arika would be the first one to get creampied by Kurt. Terena apologized, saying it was her safe day, and that she'd be careful next time. Arika said next time it'll be her (to get cummed inside?), and that after all the shit calms down, she'll go on a date with Kurt. I think that's pretty much it, if I left out any semi-important details let me know.