Strange ban

You can always submit a case review if eligible. Here is the info on how to do that: http://enforcement.xbox.com/home/howto#sub-topic0-6

You would have needed to request a review back when the ban actually happened. And unfortunately this can happen if you are letting someone else use your account. Best thing to do is not let others lot into your account and not retaliating to people even
if they are being rude. You should let you brother know he can mute and block people.

Not much you can do about it now that it's 2 years later. You would have had to contact the enforcement team at the time to see about submitting a case review to overturn your ban at the time. You can report the other person's messages since you
still have them, but I don't know if the enforcement team would consider them too old or would still take action.

Additionally, people are still responsible for whatever happens on their account.

"However, as the owner of the account you are ultimately responsible for how the account is used, both good and bad. When we issue a suspension, it’s not necessarily because you committed a violation,
but because the account was used to commit a violation. Even if you weren’t home or holding the controller when the activity occurred, if a particular Gamertag was used to harass other players or cheated in a game we have to hold it accountable"

https://enforcement.xbox.com/en-CA/home/howTo#sub-topic0-5

2017? Thats been a while, you could gwt in touch with the enforcement team & see what could be done now.... just don’t get your hopes up.

Since it's been over 1 year it would no longer be possible to get in touch with the enforcement team.

Here's the deal:

Head on over to this link
https://enforcement.xbox.com/enforcement/showenforcementhistory, sign in, then click the orange gavel. Once you're in, look next to your current enforcement action and look for a review button next to it. If you see it, click
on it and fill out the form requesting that Enforcement review your current suspension. Not all actions warrant a review as it is at enforcements discretion but, you might be in luck. This is the ONLY way to get an enforcement action overturned. Good luck!