Ok, if your uncle dies, making you oldest dynasty member, then you should be the boys hier and he should get smothered by his maid and all will be well. Give it a try and let us know how it goes.
That's exactly the case when my uncle dies. Though I have no interest in smothering those children with pillows; my house rule is to populate every duchies with my dynasty, which is why this issue comes up in the first place. The idea is that matrilinearly marrying my dynastic females prevents inheritance blobbing.