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I can't really comment on the product you mentioned above, but one common suggestion on this forum is to have someone with little to no credit apply for secured cards through Capital One and Discover. After 6 months FICO scores will be generated and shortly after the option to unsecure one or both of them will become available assuming sound credit behavior (no late payments, utilization kept low). At about the time both accounts turn 1 year in age, a 3rd card could be applied for from a company that is alright with thinner files, such as Amex.
Just so you're not surprised by this, Capital One has been telling customers that their secured cards are no longer unsecuring.
I don't see any harm in adding the Navy AU limit. I don't recall anyone reporting denials based on high limit AU cards. Of course, there'd be other factors to consider. It needs to be practical to keep utilization in check on that card. And ideally, the card is more than two years old (and not an AMEX issued by AMEX; the NFCU AMEX would be fine).
It should also be noted that banks will treat AUs differently. In some cases, they help. In other cases, they're ignored. The only way I've seen them treated negatively by a lender is Chase counting them against 5/24.
I'd suggest that NFCU should be the first place to discuss a card of his own.
5/24 is a Chase rule that results in denials if one has acquired five or more cards in the past 24 months. A lot of banks are sensitive to new accounts, but Chase is more specific about it. AUs count toward 5/24; loans do not.
It's important to think about it because your sibling will likely be positioned for a Chase card (if he wants it) when his oldest card reaches a year in age.