Jamieson, Fausset and Brown on Isaiah 65:4

4. remain among ... graves--namely, for purposes of necromancy, as if to hold converse with the dead (Is 8:19, 20; compare Mr 5:3); or, for the sake of purifications, usually performed at night among sepulchres, to appease the manes [Maurer].

monuments--Hebrew, "pass the night in hidden recesses," either the idol's inmost shrines ("consecrated precincts") [Horsley], where they used to sleep, in order to have divine communications in dreams [Jerome]; or better, on account of the parallel "graves," sepulchral caves [Maurer].

eat swine's flesh--To eat it at all was contrary to God's law (Le 11:7), but it much increased their guilt that they ate it in idolatrous sacrifices (compare Is 66:17). Varro (On Agriculture, 2.4) says that swine were first used in sacrifices; the Latins sacrificed a pig to Ceres; it was also offered on occasion of treaties and marriages.

broth--so called from the "pieces" (Margin) or fragments of bread over which the broth was poured [Gesenius]; such broth, made of swine's flesh, offered in sacrifice, was thought to be especially acceptable to the idol and was used in magic rites. Or, "fragments (pieces) of abominable foods," &c. This fourth clause explains more fully the third, as the second does the first [Maurer].

is in--rather, literally, "is their vessels," that is, constitute their vessels' contents. The Jews, in our Lord's days, and ever since the return from Babylon, have been free from idolatry; still the imagery from idolatrous abominations, as being the sin most loathsome in God's eyes and that most prevalent in Isaiah's time, is employed to describe the foul sin of Israel in all ages, culminating in their killing Messiah, and still rejecting Him.