If you take two modern species (let's denote them with A and B) you can't really say that A is an ancestor of B or vice-versa, but you can say that they are related to some extent, that is they have a common ancestor that lived X years ago. But then all eukaryotes are believed to have a common ancestor that lived an eon ago. Hence the notion of phylogenetic relevance is relative by definition. That means that all species are related, but when you take a bunch of them you can try to reconstruct the order of speciation. This can be achieved by calculating distances (e.g. genetic sequence distances, morphological distances) and applying an evolutionary model. A brief summary is that you NEED to have a sort of distance to investigate ancestry in the first place and then you clearly NEED more than 2 species for your phylogenetic constructs to make any sense.
Since I find that your statement makes little sense, obviously I can't find a reference to bake your words. But I can give you an exhaustive list of good phylogenetics textbooks for your erudition.
I believe that one thing is worth a remark. In fact it is easier to correctly reconstruct the order of speciation events than it is to precisely estimate any sort of evolutionary distance, because the distances we have are models. Simply put they approximate the number of evolutionary events that have happened since the last common ancestor of some species (or better to say the last common set of genomic and/or phenotypic traits, because speciation is a lot more than simple trait-wise divergence). From this point of view you might say that it's easier to estimate ancestry than it is to estimate distance, but then you will be over-simplifying things to such an extent, that you will drop the fact that there is no way you can speak about ancestry without having some distance estimations in the first place. The precision of distance estimations is a separate topic. So your statement will make a lot more sense if you modify it accordingly.