Should a sister tell her parents about her engaged sister's flirting with her fiancé, or should she conceal it, knowing that the younger sister fears her sister will not accept advice?

You must hasten to repent to Allah Almighty for accessing your sister’s phone and reading her messages without her permission; this is a forbidden form of espionage. If it was observed that your sister exchanged flirtatious messages with her suitor, then you must conceal her fault and advise her with wisdom and gentleness, reminding her of the limits of interaction between fiancés, while recommending caution against opening the doors to temptation.