After speaking to my dad, he said he thinks the Monte Sarmiento arrived either in the port of Rio de Janeiro or Rio Grande, he wasn't completely sure. But he was sure that my grandparents, from one of those places, took another ship to Porto Alegre. This ship's name was supposed to start with "Ita" , as with all ships used to within Brazil.
I just found it on my grandfather's passport a stamp of the Rio Grande's port. So I guess he arrived in Brazil through Rio Grande indeed.
As a funny story, my dad told me that while in this ship, his mom ate for the first time the famous Brazilian "feijoada". This traditional meal is served with a kind of yucca flour that is supposed to go with the beans, the "farinha". But his mom didn't know and put pure farinha in the spoon, and then in her mouth. The result was disastrous, as she thought it was sand, and spat the whole thing out. For those who don't know, farinha's looks and texture do resemble sand.