For pollination purposes that is. I’m thinking about picking two plants that a local backyard grower propagates from seed.
I had read that two different seedlings would be good enough, but not sure how reliable that was…and I don’t have direct experience. I mostly can’t get mine to escape the late spring freezes!
for almost all plants siblings can pollinate each other. Even if they came from the same mother/father combination.
The exceptions are when there is a certain gene/allele causing self infertility. Like the case with sweet cherries.
as far as I’m aware, nanking cherries have no such thing.