Why is it that the brothers of my boyfriend often feel seem delighted when we are together with their brother?

I notice that when I visit my boyfriend who lives on the same house with his brothers who also have their own partners, his brothers will keep looking at me and their brother and seem happy or seem like they want to tease.

I wonder why they seem that way. Our relationship though is fairly new and my boyfriend told me I'm the first person he introduced to his family after 4 years so not sure if that's the reason.

I just feel a bit bothered because it's when I'm visiting their house that they seem happy looking at me. Like i seem to be someone they've talked about.

Why is it that the brothers of my boyfriend often feel seem delighted when we are together with their brother?
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