My sister likes my ex-boyfriend, and I don’t know what to tell her. It’s not as if I have a problem if they get together, but the whole idea sounds so awkward that I don’t know how I will cope. I told her this won’t work but she says I’m just being prudish and immature. My ex is a great guy, and we broke up a few years ago only because there was no real interest in either of us wanting to maintain our relationship. We both felt as if it was too forced, so it’s not as if I am angry or jealous about this happening. It’s just the idea that makes me cringe. If they get together and it becomes serious, I don’t know what the long-term implications of this will be. How do I handle this situation without being too pushy?
Everything you are worried about exists in the realm of speculation because it hasn’t moved beyond your sister expressing an interest in your ex-boyfriend. Does he know about this? Does he have an interest in her too? Will he or won’t he think of this as an awkward and avoidable situation? If you can’t convince your sister that this is a bad idea, why not wait until something happens before thinking about what the best course of action will be? You can also speak to your ex about this if it gets to a point where you have to express your objections directly. Yes, it will be strange, but it hasn’t happened yet.

How do I know if someone is genuinely interested in me or just checking me out because he wants to know if I am worth getting to know?
When in doubt, ask questions. Do you want this person to be interested in you? Do you have feelings for him and feel the need to get to know him too? If you think there’s something here and you want to explore it, ask him out for coffee and take it from there.