Recently, my brother often sends me short messages saying that he likes me, but I dare not refuse him for fear of hurting the harmony between our two families. I dare not open it now, but ...

It's simple. Just refuse. Because feelings can't be delayed, there is a saying called cut the gordian knot. It is a very tangled thing for my brother to like my sister, so we should face it bravely. You can send him a message to explain your position, or call him directly to explain that the more you avoid him, the more you feel emotional. We should be quick, firm and accurate in dealing with creatures like human beings.