About the scene that you can trigger at the beach right in the beginning: at least in the current version it is impossible to trigger that, because even if you have a bikini then your sister will give you the most modest bikini and you will wear that (you can't stop Sammy from wearing the gift from her sister) and before you can change in another bikini your sister will simply poof and never be seen again on the beach. And this behavior is intentional because you can then read the message that this scene is currently incomplete and ends at that point.
The beach scenes for several origins are incomplete and end early (I tested bimbo and exhibitionist, not sure about the others), and if you do not have a suitable swimming outfit set in your wardrobe you automatically wear the one she gives you. But at least for the Male start, if you have already set a suitable swimming outfit, you wear that instead (and do not get any free clothing from the interaction, on top of the hundred bucks you probably spent on your own bikini). Simply possessing a bikini is not enough, you need to have gone into your outfits menu and selected one that Emile is okay with.
If what you're wearing is slutty enough, the optional beach sex scene does trigger. This requires either RNG at the market, or to have already gone to the beach on your own and met Sandy before Emile takes you there.
[tested on v0.3.2.14]
