There are 11 possible combinations.
Imagine Anna, Mara, Jenny, Tina, and Sue as numbers (1, 2, 3, 4, 5). Because we need combinations of two, we can formally write it this way: (1, 2), (1, 3), (1, 4), (1, 5), (2, 3), (2, 4), (2, 5), (3, 4), (3, 5), (4, 5). This leaves us with 11 pairs. Because it would be the same if (1,2) or (2,1), this would still mean the same two sisters have to help dad, that is why the answer is not 22 combinations. Therefore the answer is 11 combinations.