At this time of day, I would allow 75 minutes from the time your car pulls out of IAD to the time it stops at DCA departures.

Three people + 4 checked bags + any hand luggage will be really tight in an ordinary sedan. I would suggest Uber XL or an Uber SUV. A bit more than standard vehicles, but worth it if you have a lot of stuff (which you do).

Other factors which will affect your timing are whether you are traveling on a US passport and whether you have GE.

Also, make certain that you are on one ticket (as opposed to an itinerary). If you misconnect, you will be rebooked.