Well let me give you some info from my experience with everything so far. I am only in the middle of the recruiting stage, so it aint much, but it might help you.
When you go to join the Canadian Forces, you are talking about a process that takes months and months. You don't even need to speak to anyone on the phone or see them in person until 1-3 months after you originally apply. So what you can do, is just hit the application button on their website, look through the information they want from you and fill it all out, and then just wait until they contact you. That's gonna give you time to really think about things and decide if you want to keep going or back out.
If you want to back out, all you gotta do basically is email them and say you want to withdraw your application or put it on hold and think about it before going further. No hurt feelings, nobody cares, nobody is gonna judge you or get mad. It happens every single day in the recruiting world that people change their minds or need more time to think about things. So just hit the button, apply now, fill out the information for the online application, that takes a few minutes and its easy to do and will help you make up your mind if you wanna go for it.
About your uncle, it depends what trade you go for. It wont ruin your chances of joining, but it could limit your career choices. For example, I don't think you can pass Top Secret security clearance usually when a family member as an extensive criminal record. That might depend on the type of crime though. For most trades though, they don't do a background check on your family, just a basic background check on yourself.
The only ones I know off the top of my head that need top secret are:
Communicator Research Operator
Intelligence Officer
Intelligence Operator
Probably a few others as well. But for MOST trades, the only thing they do is a BASIC criminal record check, credit check and a couple other small things just on you. There are a lot of members of the Canadian Forces who have criminal records themselves.