When a king is in check with a castle can a pawn take the castle if the castle has its other castle to back him up?

It really depends on what you're asking.

When in check, the only legal moves are moves that would serve to get you out of check.

If capturing the rook with your pawn would mean your king was no longer in check, then yes, the rook may be taken. If capturing the rook with your pawn would mean that your king would be in check by another piece, then no, the rook may not be taken.