120 miles, or ~2.5 (to 3) hours by car from the airport. To get to SJO by 10 a.m. you'd have to be sure to leave by 7 a.m.
Seems do-able!

If you stay the night in Quepos, then I would leave even earlier, say 6 am to allow for any delay. Ideally, I would stay near SJ airport the night before and celebrate there.

Go a bunch 120 miles too early my rule 2-3 hours B4 US flight

ALWAYS like to be around SJO at a nice place the night

B4 my AM flight best for celebration for me

hotelvillacaletas.com last decent hotel on the coast B4 SJO

like an hour away the very prettiest.

mybeaconescazu.com

The Alta Hotel

casabellarita.com nice Boutique BNB Steve and Rita are great

Happy Travels!

Thanks! Unfortunately there are 7 of us and the house in MA is already paid, we booked it assuming everyone would be on afternoon flights. I have read that we need to be at SJO 3 hours before departure for international flight - is that accurate or will 2 hours suffice?

If they say 3 hours, better safe than sorry, right?
So, on the road at 6 a.m. gives you 3 hours (probably light traffic) to get there to be 3 hours before the flight.

You have to be checked in for an international flight to the U.S. at the one-hour mark. Period. That means boarding passes in hand and checked luggage deposited. 59 minutes isn't good enough. Standing in line at 60 minutes isn't good enough. If that Sansa flight is delayed enough to make you miss that deadline, you're a no-show for your international flight and you risk having to pay another fare at the last-minute walk-up rate. That could be very expensive, especially for seven people. Costa Rica's two domestic airlines have no ticketing or baggage-transfer arrangements with outside airlines.

What you want to do could work out just fine, or it might not. I've seen enough go wrong with schedule delays in Costa Rica that, personally, I would never chance it.

Even if everything works out perfectly, there is a good deal of stress involved in worrying about whether your flight or drive will be on time, plus a much tougher travel day. Why end a great vacation on a potentially sour and exhausting note? Plus, there are some great hotels less than an hour from the airport. We'll be staying at Finca Rosa Blanca on our next trip in January.

I thought the OP made it clear that they wanted to enjoy the company of family for a celebration their final night, and the rest of the family was prebooked.
So, the question is whether they can make it to the airport on time; the answer is a qualified yes, if they get up early and get on the road (and of course acknowledge that there might be stress).