97 Grand Prix feels liks its losing power when driving

first pressure test the cooling system for problems/leak..shuttering trans/fuel problem//service the trans and replace the fuel filter ..all for under 50 dollars..plus you will be able to ask more money for it when you sell it..

I wouldn't change the transmission fluid unless it has been change regularly. If the transmission fluid wasn't changed over the years metal and dirt and other stuff sink down into the transmission. If you change the transmission fluid, all of the metal and dirt and stuff will be disturbed and when you put new transmission fluid in and drive the car you will mostly likely destroy the transmission. I was thinking about changing my transmission fluid, but my transmission fluid has never been changed since 98 and just before I bought transmission fluid, I was warned about this and decided not to do this, the shuttering might be cause by a old fuel filter or clogged injectors,

Sounds like you may have 2 problems (or more, considering the age of the car). The stumbling under acceleration sounds like it could be a plugged catalytic converter. Given the age of the car you may be able to remove it and replace it with a piece of pipe and still pass inspections if they are required. As for the cooling system try one or even two applications of Bars Leak. That is a cheap fix if it works. With any luck at all it will at least slow the coolant loss so you can use the car till you get a different one.

Have your brother-in-law change the broken "elbow" and have a shop check your cat converter. I doubt if it is a trans problem but it is OK to check the fluid level and add some if needed. You probably will not need to drain and refill especially if you plan to sell the car. If, however, you decide to, there is an internal filter that should be replaced when doing this so have a shop do the work. This filter keeps any dirt from damaging the internal parts.