Must I tell my sister about the seriousness of what happened between us—after we engaged in foreplay—or should I conceal it and pretend as if nothing happened?

You have done well by regretting this evil act, so make it a path to sincere repentance that fulfills all its conditions. Beware of everything that calls to this sin. Rectify yourself to be a good role model for your sister and others. A Muslim must conceal their sins and not disclose their transgressions. However, your discussion with your sister about this matter does not contradict concealment, as she was your partner in the sin. Therefore, inform her of your repentance and invite her to it, and help her achieve righteousness. We hope that no sin will befall you after your repentance. We also caution against the danger of some women not adhering to the Shariah guidelines regarding their dress in front of their mahrams, as this can lead to fitna (temptation) and harassment.