Travel Itinerary help please

Skip Barcelona or Paris if you want to visit family.
If you go to Paris you can easily get to Basque by train.
Seville has an international airport but no direct transatlantic flights.
If you skip Paris fly into Barcelona or fly into Seville.

Both areas Basque and Catalonia have beaches.

I'm not one to usually say this, because I completely understand the need to fit in as much as possible, but this sounds kind of rushed.

When you say "three days," that is really only 2 days and a part of a day, because of travel. So, you would actually have only 2 full days in Paris and 2 full days in Barcelona. Whether or not that works for you depends on what you'd like to see in those places, I guess.

With a total trip of only 10 days, not including travelling to and from Europe, but including travel time within Europe, trying to go to three cities in two countries is pretty optimistic, especially if you want to fit beach time in there.

Are the flights already booked? If not, maybe consider visiting Spain and leaving France for another time.

I would drop Sevilla (too hot in the summer anyway), rent a car in Barcelona and drive to the Basque Country to visit your family there, I think it will make for a much more rewarding experience. Combining the Basque Country with Barcelona also makes more geographic sense than Sevilla (on the opposite side of the country). I would also add a night to Paris, first day will be useless with jet lag. You can then drive down to Madrid drop off your car, and fly out of Madrid. So with 11 nights you could do something like this:

Fly in to Paris
Paris (4 nights)
train? (better to fly) to
Barcelona (3 nights) - rent car
drive west, stopping at Zaragoza for a few hrs
continue northwest to País Vasco
Bilbao (3 nights)
drive south to
Madrid (1 night) - drop off car
Fly out of Madrid

Trish - Carlos just gave you a great itinerary for 11 nights. I highly recommend to consider it.

Thank you, Carlos for the great schedule. This may work!!! 👍🏻

Maybe fly from Paris to Bilbao? Then rent a car and end in Barcelona.

10 days, stick to one country.

3 days in Paris is way not enough time.
Stick to Spain, do Barcelona, Madrid (day trip to Toledo) and Seville.

I don't remember there being a major international airport at Seville.
https://www.rome2rio.com/s/Nearby-Airports/Seville

You will need to research flights. I doubt if there are flights direct from Seville to the USA.

You could go from Barcelona to Seville, then end your trip in Madrid.

Use kayak.com to check for lowest fares.