Learn Portuguese (Portugal): Diz à tua irmã que venha jantar.

Diz is the informal command (imperative) for tu. We know we need to use the informal form here because the sentence later uses tua (your informal). If you were speaking to someone formally (você), the sentence would be Diga à sua irmã que venha jantar.

Why is there a grave accent on the à before tua irmã?

That à is a contraction of the preposition a (to) and the feminine definite article a (the). You say things to someone (a), and European Portuguese usually requires the definite article before possessives (a tua irmã). When you combine a + a, you get à.

The English translation says "to come", so why does the Portuguese say "que venha"?

In English, we use the infinitive ("to come") for indirect commands, but Portuguese uses que (that) followed by the subjunctive mood. When verbs of communication like dizer (to say/tell) are used to give an order or request, they trigger the subjunctive. If you were just reporting a fact ("She says she is coming"), you would use the normal present tense: Diz que vem.

Is venha the subjunctive of vir (to come) or ir (to go)?