I find lineage-linked databases to be inadequate in cases like this, especially Ancestry's online trees. In Family Historian, I prefer to link people as Associated Persons. This allows me to record the relationship which is actually stated in the source, and I don't have to use a dummy relationship to attach them to the tree ahead of the evidence.
On Ancestry's online tree, if two people have the same surname, and the source says they are brothers, I create a no-name father with the same surname and link them that way, but otherwise, I create the person as a descendant of someone in the tree and then unlink them, leaving them in the tree as an unrelated person.
I don't like creating dummy intermediates for more distant relations, because there isn't any good way on Ancestry's online tree to flag those links as a hypothesis.
Because I do not want to mis-lead other researchers, most of my Ancestry trees are private (the exception is a public tree for a same-name family which is NOT my family, created to demonstrate why I do not think their records belong to the family I am studying).
For the cases where I have a hypothesis I want to test, I create a smaller private tree separate from my main tree. This allows me to mine Ancestry's hint system without putting a false lead into my main tree. Be sure to leave yourself some kind of note about what you have done -- Ancestry does allow you to make notes in the Tree Overview page, and on each person's profile in notes (private) or comments (public), so I encourage you to leave yourself and others breadcrumbs about what you are trying to do. The real danger, as I see it, is that by creating too many dummy people, in addition to mis-leading other researchers about what you are doing, it is easy to forget what you have done, and mis-lead yourself.
This is why I prefer to keep material like this in research notes instead of 'cementing' the relationships with links in online trees.
In one of her Barefoot Genealogist videos, Crista Cowan demonstrated how to link people's profiles to each other using Weblinks. This is useful for people linked by Association or where the relationship is not known. One downside to this approach is that Weblinks are not part of the information passed from Ancestry online trees to desktop software such as Family Tree Maker or RootsMagic during the sync or TreeShare process, so it's also a good idea to record the association in Notes (which do get synced).