Should I put a stop to it before it happens?

My 13 year old son and I've lived with my boyfriend for 10 years. My boyfriend has a niece who is 2 years older than my son. I would say they grow up together because we do things together all the times. The way I see it is they are cousins. Lately, I noticed they are really good friends. They talk on the phone... show more My 13 year old son and I've lived with my boyfriend for 10 years. My boyfriend has a niece who is 2 years older than my son. I would say they grow up together because we do things together all the times. The way I see it is they are cousins. Lately, I noticed they are really good friends. They talk on the phone all the times. When we came back from a vacation, meaning they haven't seen each other for 10 days, they instantly hugged the longest I've even seen. I see that's really some good cousinly friendship. But my boyfriend said he didn't mind "something" grow in them. Hm... I don't know what to think. Technically it maybe OK since my boyfriend and I are not married so they are not real cousins. But they also grow up like cousins. How is it like socially? Would friends and family feel weird about it? Should I put a stop to it when I see chance?
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