Breakdown of Vou a casa da minha tia.
a casa
the house
de
of / from / by (means of transport) / wearing (attire)
meu
my (m/f/pl: meu, minha, meus, minhas)
a
to / at (direction)
Questions & Answers about Vou a casa da minha tia.
Why is it a casa instead of à casa?
In European Portuguese, when you refer to someone's home as a destination, you usually just use the preposition a (to) without a definite article. Just like vou a casa means "I am going home", vou a casa da minha tia means "I am going to my aunt's place". If you used à (which is the preposition a plus the article a), it would sound like you are pointing out the physical building itself rather than the concept of visiting her home.
Could I use para instead of a and say Vou para casa da minha tia?
You could, but it changes the meaning. With verbs of motion like ir (to go), using the preposition a implies a short visit with the intention of returning. Using para implies moving there or staying for a long time. Since you are likely just visiting your aunt, a is the more natural choice.
Why does the sentence use da before minha instead of just de?
In European Portuguese, you almost always put a definite article (o, a, os, or as) before a possessive word like minha (my). Because tia is feminine, we need the article a. When the preposition de (of) meets the article a (the), they merge into da. Literally, the phrase builds as "of the my aunt".