And, like Pinkie, I see Cheese as one of the younger siblings while Mud Briar is most likely his eldest brother.
I see this particular scene taking place right after Cheese saw Pinkie’s party in Ponyville. Overcome with excitement and newfound purpose, he ran back home and threw a party for his big brother (his first party ever). And, like Maud, Mud Briar can… um… barely contain his excitement…?