When you are talking about something and somebody offers new information about it, this new information usually takes the particle が.
わ、車がきれいですね。 Wow, the car’s clean!
そお、兄が掃除してくれた。 Yes, my brother cleaned it for me.
Here, 車 take the particle が when we mention it for the first time, after which it becomes the topic and doesn’t have to be mentioned again. In the next sentence, 兄 is the new information. With the unsaid parts written out, it would look like this:
そお、［車は］兄が掃除してくれた。