I think that’s what the topics list gets us when we type it in beneath the question (also causing the question to appear in “Questions for You” lists for people who have those topics on their lists of interests). If you create two questions with the same lists of topics in both, I think they will come up in each other’s sidebars.
Note, some of those siblings questions are really old—look at the date of origin at the top—but many have more recent answers. (Answers are not going to be date-stamped, but you can always tell when a comment is written by a very recent member.) It’s ok to respond to the old ones. Sometimes a whole new spate of responses springs up, and people who are still following the question will see it and often join in.