How do I tell her I am not serious?

I have a problem with my girlfriend and don’t know how to bring it up with her. She thinks we are destined to be together for a long time and make this a permanent thing, but I have always looked at it as a casual relationship that will run its course some day. She falls more in love with me every day and I am increasingly afraid of how to tell her that my expectations are so different from hers. We have been together for two years now and I expected to end this and move on when I turned 30, which happens next year. I am worried that she may assume it will be the year I propose to her or something. Even her parents and family now treat me differently, and I suspect it is because she has told them how serious she is about me. What should I do?
If you haven’t had the courage to be honest about your expectations or open about this relationship, you can’t possibly put the onus on your girlfriend for feeling the way she does. The sooner you tell her where things stand, the better it will be for her. It feels as if your refusal to speak up comes from a place of selfishness, because you are afraid that she will end this. If that is the reason, you owe her the truth more than ever. No relationship built on mismatched expectations can last. If this isn’t serious, why are you leading her on to believe it is more than what you think it is? Stop thinking about it and speak to her at once.

What do I tell my brother about my boyfriend? They know each other from school and don’t like each other. I am afraid of what my brother may say to our parents.
You have a right to be in a relationship with another adult irrespective of what your brother or family thinks. Why not speak to your parents first, if that bothers you?