How can twins not look at all alike?

Most of the time, fraternal twins don't really look alike. In rare cases such as with the Olsen twins, they do look enough alike that they could almost pass for being identical twins.

They must be from two separate fertilized eggs.  Versus one egg that split.  Fraternal twins, I think, are rare when they’re the same sex.  

Since they are fraternal twins (two eggs fertilized at the same time) they are exactly the same as any other siblings, who may look alike or not. My sister looks like out father. I look like our mother.

It's the difference between identical and fraternal twins. Do some research to see why this is the case.

thats cause theyre fraternal twins, im a twin myself but my sister died 6 months ago, we were identical so we looked just like each other

Genetics play a major part in our features whether twins or not my twin granddaughters are not identical one lols like their father other like their mom literally and besides knowing they are twins and dress alike you wouldn’t know they were twins one is slimmer than the other also. Both are precious blessings. 

They are sisters.  Not all sisters look like each other.  Not all siblings have the same characters.  There are different kinds of twins.  Ones born from the same egg - who are identical to look at and the same sex.  Other twins are from two separate eggs so although they're born on the same day are just ordinary sisters, brothers or a brother and sister.  Like all other siblings they are born from separate eggs.  Their particular birth had 2 eggs rather than one.

"Identical twins" are the result of a single fertilized egg that divides and forms 2 identical persons.  "Fraternal twins" are the result of 2 eggs that are fertilized and develop at the same time:  there may be a strong - or no - resemblance between fraternal twins . . . . in fact, oftentimes one is a boy and one a girl.  And fraternal twins are often fathered by different men when a woman has multiple sex partners.