Looking at the banana plant in the yard, Aarav says it is a tree — it is tall and it has a "trunk"

Why

Aarav is going by the two things that most obviously make something look like a tree — it is tall, and it has a thick upright 'trunk'. But plants are sorted into herb, shrub, and tree by what their STEM is actually made of, not by height. A tree's trunk is hard and woody; a banana plant's 'trunk' is a pseudostem — a tight column of packed leaf bases, soft and juicy, with no wood in it at all. That makes the banana plant a herb (a very large one — often called the world's largest herb), the same broad group as a tomato or a wheat plant, not a tree. Test it: press a banana 'trunk' and press a real tree trunk — one gives way like a stack of wet leaves, the other is solid hard wood. His sister is right: the banana plant is a giant herb — height and an upright shape do not make a tree; a woody stem does.