Am I too petty?

My boyfriend (M29) invited me (F28) to his birthday dinner with his family. He said his mom and sister will pay for him as a birthday treat but me and his brother and sister's bf will need to pay ourselves. Me and him will celebrate his birthday the next day separately too, and i will pay for the dinner and gift. To be honest, im a bit disappointed because I thought when he invited me to the family dinner i will not need to pay. Because if i invite him to my birthday's dinner with my parents, we will definitely not make him pay at all. His mom is working but not earning much so i dont think she can pay for the whole dinner. And the rest are just around the same age as mine. But still, am i too petty to feed disappointed?how should i feel in this case? My perspective is from an Asian country.