@Sora_92 a lot of things hinting that they're siblings, like how in chapter 2 they came home together, and in this chapter the mother even ask nei dinner. the possibilities are either somehow under some circumstances they live together, or they are siblings.
@r_ryzaki In chapter 2, Souta says "sorry for intrusion" which is usually said when someone visits someone else's house. I think. They also have different family names.
Next chapter will elaborate it a bit more though that the mom who appeared here is Nei's mom. Look forward to next chapter~