Why was Ritchie....

It's strongly hinted that Ritchie and Rosie are dating when Bob arrived. When their mother hugged Bob. Ritchie had his arm around Rosie. Later when Ritchie was singing at his sister's birthday party. Their mother was looking around to see where was Rosie and Bob were at.

Bob was busy taking Rosie's cherry. As we find out after they had sex. It was Rosie's first time.

Establishes the mistrust Ritchie has for Bob, right from the get go, and uses an example where the audience can make no mistake about it, Bob taking Ritchie's girlfriend's virginity. The juxtaposition between Bob and Rosa's tryst and Ritchie singing folk songs with the elders and the children emphasizes Ritchie's kindheartedness and naivety and Bob's shrewdness. This scene sets the tone for these characters and, really, the entire movie, as in my opinion the way Ritchie and Bob play off of each other makes this movie what it is.

That was a little sad. Ritchie seemed to really care about Rosie.