I have no idea what kind of car you have, but I see that your only option is REALLY to sell or trade your current car and get a different one. CARS DON'T HAVE TO BE EXPENSIVE! Don't forget that whatever car you have you can STILL get SOME money for it- be it a down payment on another car OR the full price of another car.
For example, Honda Accords are SO reliable- you can get a used one, and still be able to use it for many years to come! Our used Honda Accord ('94) could most certainly fit 3 normal sized car seats. I'd say you could find a good car in this category for under 4,000. You can pay in full if your current car is worth that or more, or do payments, you gotta do SOMETHING, though...
Here's my story:
When we found out that we were going to have a fourth child I panicked because that meant getting another vehicle. I had always paid in full for my cars, and I knew for this one I had to do monthly payments. We decided that a minivan was the best choice out of our two options minivan or suv... We heard from many parents to get a sienna or odyssey- because they have "So many features!", and honestly, they are trendy! BUT i heard from multiple mechanics that a chevy venture had the LEAST amount of issues, and that they see them far less often than the "popular" vehicles. Well, being a woman, I thought the popular ones were cuter and cooler, but the price, and reliability on the Venture sold me on the idea. We got a 2005 Venture two years ago that was JUST $9000 (while other models were over twice as much!), and it hasn't given us ANY problems at all, I LOVE our van! I NEED to also mention that we have very little money- and we literally live paycheck to paycheck, but we had to choose the safe -and legal- option for our family.
You really need to look outside the box of options you have given yourself, do what is best... Good luck and congratulations on the twins!