A 7 year old Brat

I'm at my wits end with my son.. He is a gorgeous 7 year old boy who I love to death but he is seriously doing my head in.. He is by all reports the perfect little angel at school but as soon as he sees me he is for want of a better word a brat. It starts in the car at school and continues til bed time and then kicks off again in the morning until I drop him at school. He argues with his sister just for the sake of it (eg she will say she did painting at kindy and he will say no you didn't), I will ask him to do something simple like put his bag away and he will take 25 minutes and me asking numerous times to do it. Or if I ask him to put his bike away he has to do another lap of the yard before he will think about doing it. This morning it took him 30 minutes to get dressed for school - dancing around in his undies, running down the hall, making stupid noises. He seems to be worse if his 4 year old sister is around. I feel like I am spending every morning and afternoon yelling and fighting over everything and on the verge of tears. Am I expecting too much of him??? I know he is 7 but he can be such a beautiful boy but at the moment I don't like being around him as I get so frustrated and angry with him - this morning I threw his hairbrush on the floor (breaking it) out of frustration because I had asked him 4 times to brush his hair over a period of 20 minutes. I know he understands what he is being asked and there are no issues with ADD or ADHD or anything like that. But nothing works - I take things away from him, stop him watching TV, stop him going places because of his behaviour but nothing seems to work.. I am contemplating seeing my Dr about medication for me as I am so wound up all the time.