Fortune Island

Both my brother and I have an Xbox account (we each have our own) and I was looking to by Fortune Island for Forza Horizon 4. I wanted to know if I buy the fortune island and I get the DLC for it and use it on my account, will my brother be able to use the same DLC to get fortune island on his account or does he have to buy a different one?

If using the same xbox yes
If 2 different xboxs then you can if you are game sharing