Success with the DNA aspect of your genealogical research will be enhanced if more relatives are also tested. It can sometimes be difficult to get other family members to test and it is best to take a “soft and gentle” approach when trying to convince others to test.
Prioritizing those you should ask and those you should test is also important. Generally speaking it is older family members and ones who might not be around as long as you would like. Another factor to consider is how many generations the testee is from the ancestor on whom you are really stuck. If your great-great-grandparents are the problem and your parents and their siblings are deceased, do those great-great-grandparents have any great-grandchildren who are still living? They may even be from a branch of the family with whom you are not close, but their closer relationship to the “problem people” may give them more of the missing ancestor’s DNA to help with your problem. They are probably a better person to test than your own sibling given their closer relationship to the “problem person” on whom you are stuck.