me ( F25 ) and my Stepsister ( F25 ) still sleep together

Long story short after my stepsister, and I became adults, we moved into our own place together, One night while we were watching a movie, a very sexual scene came on, and we were both caught off guard laying in bed in nothing but our underwear. Things happened that night, and every sense we have continued the relationship, we even have boyfriends now, and still continue to be together on the side without them knowing.. it feels wrong but she just feels so right..