"My sister and I are travelling abroad."
Translation:Moja siostra i ja podróżujemy za granicę.
The cases. When you travel abroad - travel behind the border, you need Accusative: za granicę.
When you are there already, you need Locative: za granicą.
"zagranica" exists as one word (foreign countries), so actually when you come back from, it's Genitive of "zagranica": z zagranicy. But you cannot travel 'do zagranicy". This is quite a peculiar example, not really logical and very... unsymmetrical.
I have no formal knowledge about Polish but as native speaker I can show you in practice how it works:
Look: "Moja siostra podróżuje za granicę", "[Ja] podrózuję za granicę". In this case "i" connects this two sentences into one, bigger. So We (I and my sister) are doing the same thing.
"Moja siostra podróżuje za granicę", "[Ja] siedzę w domu" (I am at home). Here I am doing different thing than my sister, but you can join this two sentences into one "Moja siostra podróżuje zagranicę a ja siedzę w domu".
If it is not enough please post. I will do my best to help.
Yes. The hints show possible meanings of the word, but they are dependent on the context. "A" is the 'and' that shows contrast.
Moja siostra jedzie do Anglii, a ja jadę do Francji. (My sister goes to England, and I'm going to France).
Rabat jest stolicą Maroka, a nie Algierii. (Rabat is the capital of Morocco, (and) not Algeria.
Lubię placki, a ty? (I like pancakes, and you?)
If we say, "We are travelling with my sister," that means that there is a group, which can't and doesn't include the sister, that will be joined by the sister. Therefore there will be a least three travellers. In short the "we" would never include the sister as mentioning her again is redundant.