Tristran's little sister, Louisa, acts like a pain in the butt toward him. She teases him about his odd appearance and the weird things he says. This is apparently just the way she is: "Louisa was a born inciter, and danced circles around her brother" (2.8). In other words, it's part of her nature.
She seems like a drama queen once she grows up, too. When Tristran returns from Faerie, she starts crying but also berates him for leaving, and sighs, "a little theatrically" (10.55), in order to keep his attention. She tells him that he's "'turned into a mop-haired raggle-taggle gypsy'" (10.47), and also that he "'quite spoiled our Christmas'" (10.57). But then she hugs him "so tightly that he could not breathe" (10.60), surprising both Tristran and us.
Louisa seems to be a bit of an attention seeker, so she was probably jealous that Tristran generated so much attention even in his absence. But it's not like she hates her brother, so she is also probably glad to see him when he returns, even with all her mixed feelings.