A conflict of interest is when you (the person with the conflict) has an interest in the results that counters or competes, or appears to counter/compete, with the desire to discover the truth.
The fact that the co-authors are siblings, doesn’t in itself, cause a conflict of interest. In fact, I find it rather difficult to come up with a hypothetical scenario where it would lead to a such a conflict. At the most, I suppose that one of them might be participating in order to bolster the other for some reason, but in that case, I would assume that the sibling has an even stronger motive, which should be disclosed or obvious (confirming their own previous research).
A conflict of interest should be understood as a motive sufficient to cause you to endanger your career and reputation by falsifying data. This is frequently monetary, but it doesn’t have to be. But it has to be comprehensibly compelling. This is why companies frequently have a limit on the value of gifts to employees, because it’s not expected that someone would jeopardize a job for a single gift that was of no real value. If you are investigating the effectiveness of a drug, and the CEO of that company once gave you a pack of gum at a trade show, that’s not going to cause you to knowingly make up your results as a way to say thanks. If on the other hand, the same CEO had gotten your child into an experimental trial for the drug, and saved your child’s life, you might, out of gratitude, be willing to exaggerate the effectiveness.
The reason for the rules on disclosing conflicts of interest, are two. One, it gives the opportunity for heightened scrutiny, to find those that have let the conflict override truth. Two, disclosure increases trust, and conversely non-disclosure is superficial evidence of fraud.
If you owe a life-debt to someone or they are your primary source of income and fail to disclose that, upon discovery everyone will assume your results are both fraudulent and useless.