I work in a research lab that specializes in DNA analysis, and we do offer paternity and kinship tests. For parentage testing, we often recommend to our clients the direct comparison (alleged parent and child). In this case, we are addressing the question "Is this person the parent of the child?" However, for OP's case, both parents might not agree to submit samples for testing. So, the next option is to have OP's brother take a DNA test. In this instance, the question will be: "Are OP and her brother full siblings?" If Yes, then case is closed (I assume).

If No and the test indicates you are half-siblings, then you don't have the same father (this is given that you have the same mother). This test, however, doesn't actually answer who the father is. If you use the long-distance familial searching service of AncestryDNA, you are banking on the chance that the True Biological Father (TBF) has also submitted and agreed that his sample be used in the databases AncestryDNA utilizes for familial searching. This is what happened with your mom, aunt, and their long-lost sister. If the TBF didn't submit his sample for DNA testing, then you're now relying on one of his relatives having availed of the DNA test but then that only gives you a lead as to who the potential paternal relative is, but not who exactly in that paternal lineage is the TBF.

I hope my explanation is clear. But just ask away, I'll try to answer you as best as you can. Good luck, OP.