I'm just going to guess that these are Claire's little cousins or something that they are semi-adopting
Or that that slime that Rei adopted gained the ability to take human form and then split into two
^unless the 2nd WN had some reveal, then no. They're just two orphans that they adopted (Mei and Alea).
Also I don't recall their appearance ever being described in the novel so this might just be the illustrator doing their own thing but who knows
The second volume has a bonus chapter about the twins. How they met Rei and Claire, where they came from, their appearance, their curse, etc. If you are interested you can buy it on amazon.