Yes, this just looks like a case where inflexible tickets are just that. The best overall option may well be during one of the periodic sales for (presumably) Spain, since this can forced down the taxes, fees and surcharges. If there is an AA flight in the booking, you may strike lucky since they regularly rejig their flights. One specific thing to check is whether the booking has an option for "retained value", in other words if the option exists to use the value of the booking to a new booking. From Spain that's unlikely, but if you Google "Finnair e-ticket order", then enter your email address, you should get an email e-ticket (to be clear, I am not talking about Finnair's Manage Booking, and yes I appreciate you are not flying Finnair). This then gives the "fare basis" and that will help us to decode what the change arrangements on your ticket. But yes, it may well come back to simply you are holding an inflexible ticket.

It may be cheaper to see what your brother can do with their travel arrangements. Utimately, though, this could end up as "one of those things". In 20 years time the money will mean nothing, but the memories won't need a cost-benefit analysis.

For COVID vaccines, I presume it's an issue with PEG. You need to talk to a clinical immunologist and in particular about the Mark (et al) study in 2021. Now usually under a big excess of caution this does invite someone to go to hospital for the jab, but Pfizer in the current form now is pretty safe, and allergic reactions now are extremely rare. That said, we (in the UK, as in JCVI) no longer vaccinate those caring for vulnerable or elderly relatives since the protective impact from person A to vulnerable person B isn't that strong from person B's perspective. If someone is over 75 years then they can have the jab regardless due to the risk factors on person A alone.