TinaClarke

Yes you are if you are a" parent or step-parent, sibling or step-sibling, or an eligible partner of your relative." of him.

1. AoS for sub143 lasts for 10 years. By the time your brother's application is assessed, you would be eligible for a new one.  

2. Bridging visa to sub835 will come into effect after sub500 naturally expires.
3. In most cases,  bridging visa to sub835 would offer the permission of study, if not, still fixable.  
4. If sub500 expires and he is on briding visa, then he can work freely (if no work permission then you need to apply for it).

I assume you are saying you received Centrelink payment in the last 2 years.

If so, the Centrelink would calculate how much you have been paid and the taxable income together.

Yes, it's the same in terms of the position in the queue.