It would seem the dealer has a problem then.

At very least they need to pay you back the money the traded vehicle was worth, and then use that to buy whatever you can afford. As the dealer has stuffed up, use this as a lever to get them to do a better deal. It may be that a cheaper car will get finance because the deposit (your trade) is a higher %, and the payments can be less.

Or can YOU find finance via your bank / credit union etc. A place that already has a business relationship with you may be more willing to advance a loan.

I have a Colt 1991 A1 Compact Model. Is there a compensator out there that will fit this weapon?

If the original loan was in your name, it doesn't matter if your sister is making the payments because it still counts against your credit.Unless your original loan was paid off, it is still your loan. To answer your question, you may qualify for a second loan if your income and credit support it.