Complex sentence: Although Alice shared a close companionship with her sister, and was upset when she married Prince Frederick of Prussia in 1858, she was closest to the Prince of Wales, with whom she held a tight and intimate bond.
Split into simpler sentences: Although Alice shared a close companionship with her sister, and was upset when she married Prince Frederick of Prussia in 1858. After that, she was closest to the Prince of Wales, with whom she held a tight and intimate bond.