Ég rugla henni stundum saman við systur hennar.

I

Breakdown of Ég rugla henni stundum saman við systur hennar.

• stundum: sometimes
• systirin: the sister
• hennar: her

Questions & Answers about Ég rugla henni stundum saman við systur hennar.

What does rugla ... saman við mean as a whole?

It is a fixed expression meaning to mix someone/something up with someone/something else or to mistake X for Y.

So in this sentence:

• Ég = I
• rugla henni saman við systur hennar = mix her up with her sister

A very literal breakdown would be something like confuse her together with her sister, but that is not how it is understood in real Icelandic. You should learn rugla ... saman við as one pattern.

Why is it henni and not hana?

Because this construction uses the first person/thing being confused in the dative.

So here:

• henni = dative of hún
• it refers to the person I mistake for someone else

This is something you largely have to learn with the expression itself:

• rugla e-u saman við e-ð = mix up X with Y

That is worth remembering as a chunk.
Also, this does not mean that rugla always takes dative in every use. In other contexts, rugla can behave differently.

Why is it systur hennar and not systir hennar?