Fraternal twins happen when two different eggs are released by the mother, and each is fertilized by a different sperm. They both have the same mother, and the same father, but the sex cells (produced by meiosis) that create them are separate. The children then develop by mitosis.

Identical twins is where the already-fertilized embryo splits into two, each becoming a separate individual, growing by mitosis.

Meiosis created the sperm and egg cell in the first place, but everything after that is mitosis.

Twins can form in 2 ways:
1. The embryo can split in 2, and "identical twins' are formed.
2. Two embryos can form and grow into fetuses at the same time. Those are 'fraternal twins' - siblings with the same birthday [brothers or sisters or 1 of each].
So nothing happens til after fertilization.

Here's what I found about 'mitosis and twins':
The only exception to this is in the case of identical twins. Identical twins form from one egg cell that has been fertilized by one sperm cell. Because they form from the same 46 chromosomes, 23 from each parent, they do have the same genes. Not all twins are identical.
meiosis and mitosis read here.

In the case of identical twins, a single sperm fertilizes a single egg. The resulting zygote splits into two, and that’s how two embryos are formed.

In the case of non-identical twins, two different eggs are fertilized by two different sperm. This means that the resulting two embryos did not come from the same egg. Thus, the twins will not be identical. Meiosis and mitosis proceeds in the same way once the zygote is formed.