Over time, the meaning of terms changes!
“Senior” and “Junior” for people with the same first and last names did not always mean that they were father and son. The same goes for “2nd”, “3rd”, etc.; this was just a way for people to distinguish people with otherwise the same name. For example, one of my ancestors was known as Ebenezer Burr “3rd”. He was the son of an Ebenezer Burr; however, his grandfather was Timothy Burr as can be well established by existing will and probate records (Fairfield Co., Connecticut).
Also, here’s a link describing “in-laws” reinforcing the importance of this tip: