Sibilants – a type of fricatives, narrower and sharper than all other fricatives ([f, v, q, ð, h]) – [s, z, ∫, ζ].
Affricates – sounds consisting of a plosive immediately followed by a fricative – [t∫, dζ].
In OE there were only 2 sibilants – [s, z]. [∫] appeared in ME and [ζ] – in NE.
Affricates [t∫, dζ] appeared both in ME and in NE.