Can Apple refund a $99.99 in-app purchase made by my younger brother?

Apple may approve it, but there’s no guarantee. From its perspective, the purchase happened on your device through your account with your payment method, so it may not treat this as an account hack. The fact that someone else in your household made the purchase doesn’t automatically require Apple to refund it.

You generally can’t add a personal explanation after submitting the initial request, but if it’s denied, there may be a one-time appeal option. The quick timing, new app, and lack of previous refund requests could help, so keep an eye on the request status rather than assuming it’s automatically rejected.

Change the password immediately, and consider requiring a password or biometric confirmation for every purchase. When letting a young child use the phone, Guided Access or similar restrictions can prevent them from leaving the intended app and making purchases.

Unfortunately, sharing access to the phone or account can make the purchase look authorized, even if you didn’t personally approve it. Still, submitting the request promptly was the right move. If it’s declined, explain that it was an accidental purchase by a child and use the available appeal process.