Well you can't rule out that the beach that they are enjoying the summer is either very remote, or their own inherited private summer home for a family getaway. From the background story wise, Neeko's Dad seems pretty well off enough to let Neeko splurge on gachas and such, so the private summer home is plausible. Perhaps it is as planned by her parents knowing Neeko seems rather at home whenever she is with her family and her few close friends, but awkward and almost xenophobic around strangers. Nevertheless she could use some sunning which is beneficial to her health and tone down on the alcohol consumption.
Oh and that almost hesitant v-sign made by Neeko's right hand seems to tell that she's feeling a little uneasy out on the beach, as opposed to Imoko who seems really exuberant enjoying the summer.