So - you're 13 and about to move from prep to senior school. You have to choose one ML to take alongside French. (Grammar reasonably advanced so far, vocab somewhat neglected ...) After yr9 you can carry on with both to GCSE or drop the new ML. (No late changes of mind possible.)
The options are Spanish, Italian, German, Russian, Japanese, Mandarin.
You like, and will continue with, Latin. You're neutral (and damn lazy) on Greek. If your choice doesn't work out you'll have only one ML GCSE / IGCSE (not certain which exam atm.) And you're currently 13 less unenthusiastic about Arts than Sciences so languages are more likely to feature in your future school career.
Your family are dithering. Pros / cons blah blah blah ...
Please advise.