My boyfriend always makes his sister more of priority than me...

I know, to him, his sister is more important than me ...

There are five members in his family : his father, his mother, himself, and his two sisters.
He is an elder brother of two pretty sisters, and they both have boyfriends already.
When he is with me having a conversation, he ALWAYS mentions his sisters.
For example, we were talking about food and which restaurant had the best cook, he said that his youngest sister could cook that dish too, and he made fun of me that I could not. (I know he didn't mean to hurt my feeling or to compare me to his sister, but I know he deeply felt something from doing so.)
Another example is about his middle sister, who is very pretty and smart. He talks about her all the time!, like, "my sister went there too" , "my sister has told me before", "my sister is very pretty today in her blue satin dress", etc. I know how much he adores his sisters because they're pretty, but it really makes me feel unimportant and inferior.
I have sibling so I understand how important his are, yet I'm sick of being on the back burner. I know he's been with his sisters longer than he's been with you, but I just feel (T_T).

On his birthday, I was invited to join his birthday dinner with his family. I gave him a gift box of genuine branded leather belt with a matching keyring. Her sisters gave him a long-sleeved shirt. He said to me, "Thank you, honey; they're beautiful.", while he said to his sisters, "Thank you, dear, I really like this shirt". And he stood up and tried that shirt on... (*.*) ... yay, without noticing another box from me. (I told my close friends about this and they said that he should have tried on the belt.)

This evening, while my bf and i were having dinner at the restaurant and about to see the movie, his sister called and told him that she had a bit headache and she was going to see a doctor WITH her boyfriend. My bf said he had to go home immediately to see her sister. Then he went home and left me there with the movie plan.
I know that his sister had a headache and she was going to see a doctor, so he - as her elder brother - had to go home. BUT she was with his boyfriend already and I swear it was just a mild headache.

Am I selfish? ...

I just really feel less important than his sisters. And I do feel it.