Here's what I understood.
Momo's jealous of Sumire's because of her commitment with painting (she also thinks Sumire's good if what the boy said is true) , since she's not committed to anything (relashionships or hobbies - from underwear's to people she doesn't like to feel hemmed) . Also she's jealous of the teachers interest in Sumire's strange habit of destroying all pictures she paints. Thus causing some envy and hate both feelings Sumire says are allien to her. Kinda "wish I could draw like her and there she goes yapping nonsense with difficult words and burning her painting like they were nothing". So on the other side she admires Sumire's paintings and also there's this sense both are lonely, have trouble connecting with people and
Sumire on the other hand seems to be paying attention to Momo's shenanigans, she reacts harshly when the boy speaks ill of Momo's and knows she has an affair with the teacher. Also she seems to be aware Momo's jealous of her. When they're talking in her room she is drawing a princess doll as a kid, as they finish talking she looks at Momo and I understood this means that in Sumire's eyes Momo is a princess but, as she stated previously, not only flesh so there must be more to it, if Sumire has difficult feeling things and expressing them, in that sense Momo does it easily, thus surpassing Sumire. I think That's why she didn't burn the painting in the end and it kinda also mean something more and makes clear that Sumire's not interested in least in the teacher (so if she has an attachment with that place and we only see Momo there there... ). So in the end Sumire's states that affairs are bad and Momo agrees and they leave together... Since sumire almost doesn't speak, I feel it's very meaningful she's talking about it to Momo. Also for someone who dislikes being attached to something searching up and down for someone else indicates some feelings too.
Well... in the end they're both weird and have a weird bond.
