Thinking of the above comments on step-siblings, I think that it is only ok if they are ~16-18+ and did not know each other when they are younger. The best acceptable scenario would be if you knew and were dating that person before joining their family as a step-sibling. However, if you became part of the family younger than the above mentioned ages, then it is not ok. Franziska was 1-2 years old when Edgeworth got adopted, so it is in no way remotely alright. Personally, the EdgeworthxFranziska ship is one of my most hated as the (platonic) older brother younger sister interactions are my favorite. Putting family situations as romance always makes me feel icky and can only be accepted under super specific conditions (such as the Yuri manga Citrus).