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Hard to say what is going on. You do have a very thin profile (meaning very few accounts) and credit scoring models can get very jittery when that is the case.
One thing that happened is your Average Age of Accounts went from 40 years to 20 when you added the new account. That's part of the loss. Your Age of Youngest Account went from 40 to 0. That's another part of it.
Congrats to you for adding your own credit card. That is a great first step.
Would your Mom be willing to pay off her Macy's card altogether? Pay it down to $0? That would help your score a lot. (And hers in all likelihood.) Then you could use your new credit card for a small purchase once per month.
BTW, if your mom were to pay off the Macy's card, it doesn't mean she wouldn't be able to use it. It's possible to use a card a whole lot and still make it report $0.
It's possible but the dent will be small. Do what makes financial sense rather than credit score sense. And remember that as the Macy's card gets paid down your score will prove quite a bit.
Are you certain that the 0% offer will last long enough for your brother to pay it off making minimum payments? And have you looked into the consequences if he cannot? Some 0% offers result in you owing interest on the entire amount if it is not paid off in (say) 12 months.
Bear in mind that you are a primary owner on this card -- not an AU. If late payments are made, those will affect you for seven years.
Sounds like you know what you are doing. Best of luck!