The baby is adorable, though the cutest thing might be Eli devolving into a nesoberi.
Umi's sister does have something of a point, though it may be the familial straightforward nature talking there — that she, like Umi, has no guile and therefore no time for it in others — and she's speaking from a position of straight privilege.
My theory on what she whispered — based on Eli's reaction — is something along the lines of "Please don't hurt my sister".
