Anyone eligible can get a Presidential based upon their parent's service, however, for the USAFA at least, only 100 appointments can be charged to that nominating source. Unlimited in number but with only 100 of those getting an appointment based on it. That's why you absolutely should apply to every nomination source you are eligible for, it's by no means any type of guarantee.
If you have an LOA - then that changes things - you'll probably receive an appointment if I understand what I've learned from the moderators and regulars here. But I'd defer to them on the question.
My son has one, but we've applied to every source we could, it just makes sense in this environment and with the knowledge that so few appointments are made from possibly many hundreds of noms...
Best