If they are identical twins, then Yuuya would have identical or almost identical genetics as Yuuma. That would make him closer to Yuuta's biological "mother" rather than half-sibling.
Half-sibling with Yuuta would imply that 50% of Yuuya's "maternal" DNA would be made up of DNA from Yuuma and 50% would be made from a completely unique source.
As siblings, Yuuya and Yuuma would share anywhere from 0% - 100% of their DNA with a mix of matches from both maternal and paternal chromosomes.
I can't remember if they are identical twins and/or if secondary genders is relevant to whether they are identical (I've read series where it is and where it isn't), but even normal full siblings wouldn't be mistaken for their niece or nephew's half-sibling due to gene distribution. ¯_(ツ)_/¯ Interesting question, though!
for once i think this is a really interesting plot for an omegaverse
(also, correct me if i'm wrong but i think genetically, yuuta is closely considered yuuya's half brother than nephew since yuuma is his identical twin)