Is it possible for a woman to conceive from two different men and give birth to half-siblings?
Yes, this is possible through something called heteropaternal superfecundation (see below for further explanation).

Of all twin births, 30% are identical and 70% are non-identical (fraternal) twins.

Identical twins result when a zygote (one egg, or ovum, fertilized by one sperm) splits at an early stage to become twins. Because the genetic material is essentially the same, they resemble each other closely.

Typically during ovulation only one ovum is released to be fertilized by one sperm. However, sometimes a woman's ovaries release two ova. Each must be fertilized by a separate sperm cell. If she has intercourse with two different men, the two ova can be fertilized by sperm from different sexual partners. The term for this event is heteropaternal superfecundation (HS): twins who have the same mother, but two different fathers.

This has been proven in paternity suits (in which there will be a bias selecting for possible infidelity) involving fraternal twins, where genetic testing must be done on each child. The frequency of heteropaternal superfecundation in this group was found (in one study) to be 2.4%. As the study's authors state, "Inferences about the frequency of HS in other populations should be drawn with caution."
