Ok I've given this some thought, because I think if EK did go away with the boys only on Thursday, and return on Sunday, it certainly looks off in the overall scheme of things, that Mariah can then be reported abducted that very night. How usual would those two occurrences line up like that is my way of looking at it.
So, why do LE ask for sightings of the family from Sunday? I'm assuming that this family had no contact with anyone outside of their own number (KH, EK, & children) on that Sunday. So LE have asked the boys. The boys have told them they were all together on Sunday, maybe in truth, maybe under pressure. There is no point in asking the boys if they saw Mariah between Thurs and Sun because they were away (assumed from photo without Mariah). LE is alive to the possibility that they are speaking under pressure and wants to know if anyone can verify it.