Do I half to buy the same brand tires

No you would not have a problem switching or mixing to other brand tires. With a AWD vehicle long as you get the same size tires for the front and rear of the car.

Buy the same brand in the correct size for both ends of the vehicle. This will eliminate any damage or issues for odd sized tires due to differences in sizes between manufacturers. Always replace all 4 tires on AWD vehicles.

I would say as long as you have the same size and brand on either axle, you should be ok. Say, GoodYears on the front, and Continentals on the rear, as an example.

Different brands such as GoodYear and Continental can have differences in dimensions even if they are the same size. Are you willing to take a chance damaging the AWD system or causing errors by using different brands on the same AWD vehicle? I would not be willing to take a chance.

My 2013 CTS coupe, performance edition, has 235-50-18 on the front and 265-45-18 on the rear, if yours are the same as mine, yes you can buy from different manufacturers. It wont make any difference. All of the above answers were for the same size tire. This Cadillac has offset tire sizes.

My question is : My CTS coupe also has different size on the back then the front, but the front has worn in the center to 4 on one side and a 1 on the other tire. The back tires are still about 6 all the way across. Do I have to buy 4 or can I get new ones for the front since they are one size, and let the back ones wear a little more ?

If your car is all-wheel drive you must have the same circumference tires on all four corners. Individual brands have different circumference with the same size tires. Not all 255 45 are the same circumference even if the tire size says so. This circumference is so critical each tire must be within 2/32 in tread wear. Following some of these opinions I'm reading here you'll find yourself with drivetrain problems