B1
one of two children who are born to the same mother at the same time
Dictionary examples:
Being a twin is great. You always have someone to blame! (0.0)
My twin ate my homework. I guess we really do think alike! (0.0)
My twin and I switched places at school. Even our teacher couldn't tell! (0.0)
a twin sister (13.2)
My sister has twin sons. (14.8)