Sibling testing
Full and half siblings share different proportions of DNA, so a sibling comparison estimates which relationship the data supports. Results are clearest when both siblings and, where possible, a known parent are tested.
Grandparent testing
A grandparent comparison can be a strong alternative to paternity testing when the alleged father is unavailable. Testing both paternal grandparents gives a stronger result than testing one.
Choosing between them
It depends on who is genuinely available and willing to be tested. A coordinator will map your family situation and recommend the comparison with the best chance of a clear answer, including whether adding one more relative would materially help.
Explore relationship DNA testing or ask via contact.