If you can really forgive her and try to have a nice time despite everything else, then what minmooch said just above. I'd ask the family there to find a flight from your US airport to their airport, as they might have different prices / websites and probably cheaper than the options you found. However, I wouldn't plan any excursion on the days you are driving the 4h. Plan to stay at a nice place, maybe some nice hotel, cottage or somewhere where you can fully relax and sleep, yet enjoy the landscape or food. You'll be extremely tired after the flight and the drive. You'll all have jet lag. Maybe start a couple of weeks earlier to change bed time times, so when you get there, the shock is less? Thinking of things such as putting them in bed earlier / later a bit each day.
If you cannot forgive and you are going to be upset... I think what this previous poster said is spot on:
"Isadorabubble
Problem is, by the time you get there with the hassle of all the travel you're going to be so p*ssed off that it's likely to end in a huge row. I think you should definitely try to have a discussion with your sister about it to see each other's points of view before (if) you go to try and alleviate any tensions"
I think you'll arrive very tired, you will know there will be no time to discuss this after the wedding and might have an argument as soon as you get there, due to tiredness and in response to questions such as how was the flight? That would put a mood to the day that won't be forgiven by any part and will make the trip completely pointless. I'd try to see your options again regarding changing flights or dates. Maybe move them to 2019 as initially intended. Ask the flight company, check their website too for small print, there might be a small charge to pay but compare it to the money you still have to spend once you are there, it might still be worth it. Check travel insurance too.
I did last year a trip to Spain with my DC, 1.5 and 3.5... I could have bitten anyone by the end of the day. It was an 1 drive, 3h flight, an 1 to change flights, a further 2h flight, an 1 drive. One of them was sick on the flight, the other one didn't want to sleep. We had too much luggage, too heavy: remember one of you will be dragging a pram and a changing bag, don't take one big wheelie suitcase each, plus hand luggage... The horror. If your eldest has a suitcase, Truki is good, don't be like us (we put her toys in), but put the flight essentials and have the child ride it / carry it: bottles, wipes, nappies, change of clothes for kids, dummies, snacks, maximum one toy. The Truki was the only suitcase that was small and easy to access, yet it was full of pointless things!