In your scenarios, all of the plaintiffs are related to the accused in a similar way and would have a common incentive to make the accusation. That kind of situation naturally raises concerns about collusion or bias. By contrast, when multiple independent and unconnected individuals come forward with similar accusations, the evidentiary weight is very different. Courts recognize that corroboration from unrelated parties strengthens credibility, because it reduces the likelihood of a coordinated or self-serving motive.
For example, in United States v. Bailey, 581 F.2d 341 (3d Cir. 1978), the court noted that corroborating testimony from independent witnesses could significantly enhance credibility and probative value. Similarly, Federal Rule of Evidence 404(b) allows prior bad acts to be admitted in limited circumstances, precisely because independent, consistent reports can establish patterns that are unlikely to result from mere coincidence or collusion.