If I download stuff from the internet does it affect the internet connection of all the computers in our house?

[marcus]

Depends on the type of internet connections you use - as well as the equipment involved in connecting to the internet. Using a wireless connection will be slower than a landline connection, but when you have to start using a router to cater for more than one connection in the same house then obviously demands are made on the router to allow for more than one user to have data continuously relayed from one computer to another. Several other factors are included - most of these are to do with network topologies (they have funny little names like 'Bus', 'Ring'. 'star' etc),
It is feasible to think that maybe your laptop connection to the internet using that router is making things slower, but then your brother being on the internet is only likely to have the same effect on you! If you are both using a wireless connection to the internet, you need to get closer to the router in order to achieve a better signal and a better transfer rate as a result.