half-brothers or sisters. people who share one
parent, but not the other. eg, people with the same mother but different fathers.
you could also
spell or pronounce it "halvsies", but you'd look like a retard.
Girl1: "I thought you had a brother. How come I never see him anymore?"
Girl2: "My
sister and I live here with our mom, but our halfsies live in Texas with their
dad."
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Guy1: "Megan and Ryan are spending a lot of
time together. Are they dating?"
Guy2: "No way, man! They're halfsies!"