You could say something if you’re concerned - 16 seems too young for a 25 year old. She should finish school or be at least 18 before dating. There’s perhaps too little information to go on here. Other relevant factors might include where are the parents here? What country is this? Has he dated other sisters? Has she dated other brothers? How sure are you that he is interested in her? What does the phrase “has her auxiliary pioneering” mean? He put her up to it? How do you know?
I don’t think it could harm if you expressed genuine concern reasonably. Why not talk to her parents rather than an elder? If you make it clear it’s genuine concern, and not some kind of congregational action, it might be received okay. Then again, maybe a relevant safe-guarder would be more appropriate than a parent, if any exist in the congregation.
Do you have a trusted person in the congregation you can ask if they view the situation similarly, or if they have more information, or a different perspective?
Maybe there’s a qualified person here who can give a better answer.