My husband was raised by his older half brother. We have been told throughout the years that he is actually my husband father.We ran an ancestry test with his daughter. Results were sharing 1249cm over 38 seg. She also has had chemo this past year. Just trying to find a more definitive answer.
I think you have two questions in one.
First, you need to learn the way half-siblings match work. If the shared DNA strongly shows a possible positive result, there you have the response.
Check this response: Distinguishing half-sibling, uncle or half nephew using Ancestry DNA?
If there's no match, but the results are still confusing, you can point to the second question: does chemo affects DNA test results in any way? I'm sure that they don´t, but not an expert.
Found another answer here where a user brings some notes about DNA test companies´ advice in case of chemo.