Complex sentence: The seat of the Dukes of Northumberland is Alnwick Castle, in Alnwick, Northumberland; their London residence is Syon House in Brentford which replaced as their London residence the demolished Northumberland House in the Strand.
Split into simpler sentences: The seat of the Dukes of Northumberland is Alnwick Castle, which is located in Alnwick, Northumberland. The family's London residence is Syon House in Brentford, which replaced, as their London residence, the demolished Northumberland House in the Strand.