Learn Hardcore Italian: Né io né mia sorella possiamo venire stasera. - Neither I nor my sister can come tonight.

It’s a correlative negative conjunction meaning neither ... nor .... It negates both coordinated subjects: né io né mia sorella = “neither I nor my sister.” Together they function as a single (plural) subject.

Do I need to add "non" as well?

When the negative element(s) come before the verb, you don’t add non: Né io né mia sorella possiamo... is correct.
When the negative part comes after the verb, you must use non: Io e mia sorella non possiamo né venire né restare.
With postverbal subjects (more natural in 3rd person): Non verranno né mia sorella né mio fratello.

Why is the verb possiamo (and not posso or può)?

Because the subject is effectively noi (“io + mia sorella”). Italian verbs agree with the overall subject:

io + someone else → 1st person plural: possiamo
The verb is the present indicative of potere: posso, puoi, può, possiamo, potete, possono.

If it were "né tu né lui", which verb form should I use?

Use 2nd person plural because “tu + lui” = voi: Né tu né lui potete venire. More patterns:

Né tu né io possiamo venire. (includes “io” → 1st plural)
Né Marco né Lucia possono venire. (3rd plural)

Can the verb ever be singular after "né ... né ..."?

The safe, common choice is plural. Singular appears in set phrases like Né l’uno né l’altro è venuto (treating the pair as a single alternative). With names or common nouns, prefer plural: Né Marco né Lucia sono venuti.

Why is there no article before mia sorella?

With possessives before a singular, close family member, Italian usually omits the article: mia sorella, mio padre, tua madre. Exceptions:

With loro you keep it: la loro sorella.
If modified/diminutive: la mia cara sorella, la mia sorellina.
In the plural, you use the article: i miei fratelli, le mie sorelle.

Why mia and not mio?

Possessives agree with the noun they modify. Sorella is feminine singular, so mia. Use mio with masculine singular nouns (e.g., mio fratello).

Do I have to repeat né before both parts?

Yes. You place né before each coordinated element: né A né B (and with longer lists: né A né B né C). Saying only one né is not standard.

Is the accent on né required? Which accent is correct?

Yes. Write né with an acute accent (é) to distinguish it from ne (the clitic pronoun “of it/from it”). The form nè (grave accent) is a spelling mistake.

Could I say "Io e mia sorella non possiamo venire stasera" instead? Which is more natural?

Yes. Io e mia sorella non possiamo venire stasera is very common and neutral. Né io né mia sorella... is a bit more emphatic, like English “Neither I nor my sister...”.

What changes if I move the "né ... né ..." after the verb?

To coordinate complements: Non possiamo né venire né restare.
To coordinate subjects after the verb (common in 3rd person): Non verranno né mia sorella né mio fratello.
For “we,” you’ll usually keep the subjects before the verb or switch to the neutral version: Io e mia sorella non possiamo venire.

Why is it venire and not andare?

Italian uses venire when the movement is toward the place of the speaker/listener or the event being considered (“come”). Use andare for movement away from that point (“go”). After an invitation, venire is typical: “Can you come (to my place/event) tonight?”

Why isn’t there an “a” before venire (like “possiamo a venire”)?

With modal verbs (potere, dovere, volere, sapere = know how to), the infinitive follows directly without a preposition: possiamo venire, dobbiamo studiare, voglio parlare, so nuotare.

Is stasera the only way to say “tonight”?

You can also say questa sera (a bit more formal/spelled out). Stanotte means “tonight/overnight” and refers to the night hours. All are correct depending on nuance.

How do you pronounce the tricky parts?

né: one syllable, closed “e” (like “ne” in “net” but without the final consonant), stressed.
possiamo: pos-SIA-mo (double “s” as a long /ss/; stress on “-sià-”).
sorella: so-REL-la (double “l”).
stasera: sta-SE-ra (stress on “se”).

Is there a more compact way to say “Neither of us can come tonight”?

Yes: Nessuno di noi può venire stasera. If both people are female, you can make it feminine: Nessuna di noi può venire stasera.

Can I drop the pronoun io since Italian often omits subject pronouns?

Not here. You must name both elements of the coordination. Né mia sorella possiamo venire is ungrammatical. You need both: Né io né mia sorella possiamo venire.