Is it love or friendship or flirt

Hey Friends,

I am Tina, basically telugite but born and brought up in Kilvelur in Tamilnadu, working in bangalore.

I am here in bangalore since 2 years, and one thing i am not able to understand about the relationship between few guys and girls here..
I know one guy, who met a girl, then started roaming with her. That guy is my friend's brother. And after started roaming with that girl, he was totally disconnected with his friends circle, and he seldom worried about it. When my friend advised him not to do such things, he even broke up with him , too. Being brothers, they stayed in different houses, and seldom visited each other. My friend told that that girl even stays with this guy. Whenever we ask my friends brother about the kind of relationship he shares with that girl, he always says "very close friend", i seriously dont understand why he is spoiling the relationship - friendship. This is not the only case i have seen, but i have seen a couple.

I seriously couldnt digest, i dont know whether i am too backward, since i am brought up in town, but definitely my intutive mind says that it is wrong. Friends, what do you say?

Tina