23AndMe provides the following table to summarize the average percent DNA shared for different types of relationships according to their simulations. You may notice that several relationships share the same average percent DNA; this can account for a predicted relationship of aunt/niece for a pair of half sisters.

Relationship Average % DNA Shared Range
Identical Twin 100% N/A
Parent / Child
Full Sibling
50% Varies by specific relationship
Grandparent / Grandchild
Aunt / Uncle
Niece / Nephew
Half Sibling
25% Varies by specific relationship
1st Cousin 12.5% 7.31% – 13.8%
1st Cousin once removed 6.25% 3.3% – 8.51%
2nd Cousin 3.13% 2.85% – 5.04%
2nd Cousin once removed 1.5% 0.57% – 2.54%
3rd Cousin 0.78% 0.3% – 2.0%
4th Cousin 0.20% 0.07% – 0.5%
5th Cousin 0.05% Variable
6th Cousin 0.01% Variable