Fireheart (later Firestar) is a leader, and the Warrior Code strictly forbids relationships between leaders and warriors. Sandstorm, though she loves Fireheart, understands and respects the code, and they never act on their feelings.
They do, however, become close mentors to Firestar's kits, Squirrelpaw, Bramblepaw, and Tawnypaw, who are born to Firestar and his mate, Sandstorm's sister, Spottedleaf.