Here's my interpretation:
Preserving structure while modernising and clarifying as much as possible,
"Heaven and earth! Are the shades of Pemberley to be thus polluted?"
Heaven's above! (a softer form of "Oh God!") Is this how the family line of Pemberley will get corrupted? (referring to intermarriage with someone deemed unworthy)
(Original reply follows)
There seems to be a very thorough answer here.
But in a nutshell, I think it's as @Vilmar suggests, though perhaps more another way of saying that the family (via the evocative use of long dead ancestors through 'shades') will be tainted by the intermarriage.
EDIT: Why do I get the sense the speaker is worried that the family line will be tainted in the near future? Because Heaven and Earth! seems to be a statement of dismay or worry, like "goodness gracious!" or "Gosh no!" And the phrase to be indicates the future; thus polluted, tainted.