Not really what your criteria is being that I’m a pro life libertarian as far as ideals I align with most on what you’re looking for.
Even though I am religious, my argument against abortion is firstly a scientific one then on moral principal second. On the science side it’s a human from the moment of conception. On the moral side it’s that I believe all humans deserve human rights no matter at what stage of development there are. Just as soon as you make exceptions to kill for one type or subset of humankind you open the door to others. Usually this is done by labeling a certain group as not human to justify oppression of said group. Terms usually used to justify acts of violence against other humans are property, subhuman, animals, savages, clump of cells, parasite, etc. Usually for libertarians it boils down to having a code called the non-aggression principal which is essentially don’t fuck with other people. This is also why I’m anti capital punishment.
I hope that helps. Also, good luck at your family get togethers, lol. It feels like you’re looking for ammunition for debates.