Why is the first thing people try to do is start a 50 year old car that has been sitting for 15? It got parked for a reason. IF this thing has been in the family why was it parked? Even if it starts now what? The tires are shot, the brakes are shot, every fluid that was in it is now CRAP. You can do more damage just seeing if it will fire than investigating problems and rooting out causes. Early 70s Charger will not start. OK. Put the car up on jack stands and get to work. ALL fluids need changed. The motor at least needs the VCs off and a little lookie liew. Seeing if the motor will crank is ok but like was said Yank the plugs out and give the cylinders a squirt of something slick. IF the motor turns over great then proceed to the basics. Suck, Squeeze, Bang, Blow. Do we have fuel at the carb? Yes Proceed to ignition. Pretty easy to check for spark in all the places it should be. GIven that it MIGHT start. Just take your time and stop throwing parts at it. IF your BIL is a clueless cheapskate then you are right there is NO TELLLING what he might have done.
Books are your friend. IF you are going to try fixing the car up over time buy you a Service Manual for your car (Amazon has them I am sure). It is the best money you will spend on any car that you are not familiar with.
Happy motoring.
Todd