The best conditioner for brittle hair is one rich in protein. Protein helps repair the hair's structure and strengthen the strands. It also works as a temporary sealant for hair so the split ends bond back together.
It is important to be careful when using a protein-based conditioner because too much protein is not a good thing. Look for protein farther down the ingredient label because that means the conditioner contains adequate protein, but not too much. It is also important to ensure the conditioner provides a lot of moisture, or the hair can become more brittle.