Can wireless (in my home) be broken in to.

My brother has cables running everywere in the house cause he is scared to use wireless. He thinks someone will break into his computer and take his files or what ever he has on his computer if he has wireless internet. My question is this
1.Can someone break in to your wireless network
2. can they take your files
3. can they control your computer
4 how long would it take then to hack the secure code.

I set up wireless for my self in the house cause I have a laptop. not a desktop.