@greatninja3 maybe they have, I mean the younger somehow knows about there being new half siblings. It's also possible that they're avoiding each other. Or, they aren't sisters at all and we got baited.
@Kalamel I'll wait for confirmation. It's pretty weird for this two to haven't met each other in their school. But again, it's already weird that mom was able to go into school with her son. Like how did she know he will be going there?