Hi L.,
Manners are very important to me.... second only to family harmony and at home intimacy.
Your little ones are still at the age where certain things are funny at home, but will not be so funny in public. Your oldest has begun to realize this.
You can let your burper/gassy girl know that she may think it’s funny, but it hurts her sister’s feelings (she probably doesn’t understand the concept of embarrassment yet).
You can also tell her “ever so gently”, that someday (soon) she will do this outside the home and her feelings may be hurt, because burping and passing gas in public, is just NOT nice. Sometimes you can’t help it, but most of the time YOU CAN!
Ask her to practice at home, by “asking to leave the room, be excused”, when she thinks she needs to do something private. If she makes a game out of this or abuses being excused.....THEN we have another topic to address.