You assume they were siblings because there's no reason not to?
First, thank you for admitting your assertion is based on an assumption.
Second, the reason no one should assume Jesus's brothers (kinsmen/relatives) were siblings, or any type of family member, is because the familial definition of "ἀδελφοί" (adelphoi;brothers) is "kinsmen, or relative," and thus could refer to not only siblings, but cousins, nephews, and uncles, etc. So, if you want your baseless assertion to evolve into a substantiated fact, then you need to provide information that shows Jesus's brothers (kinsmen/relatives) were siblings.
Finally, I've actually given something (scriptural verses and early Christian testimonies), which is better than your nothing, to show those kinsmen/relatives of Jesus were the sons of Jesus's uncle, and thus His cousins. Another thing you don't do is actually show why that evidence of mine doesn't show what I claim it does.
Lol, thank you for demonstrating you don't understand how's there's more than one word that can be used to refer to a cousin and other types of family members. Also, you also just explained something what I've been trying to get you to understand, and which you've been fighting me on, and you don't even realize it. In fact, you think it supports your argument xD
Lets see if you can pin point what it is on your own first. If not, I'll show you.
Look to the Aramaic-- you know? The language they spoke and used, rather than the language the scriptures were translated into (Greek)
There is a word for brother---- "aho" which rendered into Greek becomes adelphos
There is also a word for cousin-- and it's specific to son of my father's brother. (son of my uncle) "bar dodo."
Lastly-- and with this I'm done here..... there is still another Greek word that specifies "cousin" that could/should have been used if that's what was meant-- and it's anepsios -- not adelphios.
Good luck with your studies and your thesis. You'll need it.