Learn Hardcore Spanish (Spain): Mi hermana no friega ahora, sino que mete las tazas en el lavavajillas. - My sister is not washing now; rather, she puts the cups in the dishwasher.

Replacement after a negation: No friega ahora, sino que mete... (She isn’t doing X, but rather Y.)
Mere contrast: Friega a menudo, pero hoy mete las tazas en el lavavajillas. (She does X, but today Y happens too.)

When do I use sino vs sino que?

sino + noun/adjective/adverb/infinitive: No quiere café, sino té. / No vino para quedarse, sino para saludar.
sino que + conjugated verb: No friega ahora, sino que mete las tazas...

In your sentence the second part has a finite verb (mete), so you need sino que.

Can I say si no instead of sino?

No. sino = “but rather.” si no = “if not.”

sino: No llamó, sino que envió un mensaje.
si no: Si no llama, me preocuparé.

Is No friega ahora the best way to say “she isn’t washing dishes right now”?

It’s grammatical but ambiguous. The simple present can mean “now” or “these days.” For an action happening right this moment, Spanish usually prefers the progressive:

More specific to “right now”: No está fregando ahora, sino que está metiendo las tazas...
Habitual/current period: Ahora no friega; mete las tazas en el lavavajillas.

How is fregar conjugated, and what’s irregular about it?

It’s an e→ie stem-changer in the present.

Present: yo friego, tú friegas, él/ella friega, nosotros fregamos, vosotros fregáis, ellos friegan.
Gerund/participle: fregando, fregado.
Spelling change in preterite yo-form: fregué (to keep the hard g sound).
Imperatives: friega (tú), no friegues (tú), friegue (usted).

Does fregar always mean “to wash the dishes”?

In Spain, yes, fregar (los platos) is normal for washing dishes and also “to scrub” surfaces (fregar el suelo). In much of Latin America, people prefer lavar los platos; fregar can mean “to scrub” or colloquially “to annoy.” In Mexico, fregar can sound rude in some contexts. For Spain, your sentence is perfectly natural.

How do I pronounce the tricky words here?

friega: roughly “FREE-eh-gah” (stress on FREE; g as in “get”).
sino: “SEE-no.”
que: “keh.”
mete: “MEH-teh.”
lavavajillas: “lah-vah-vah-HEE-yas” (j like a harsh English h; ll usually sounds like English y in Spain).

Why use mete and not pone?

meter implies putting something into an enclosed space: meter las tazas en el lavavajillas (to load the dishwasher).
poner is more general (“to put/place”) and in Spain also means “to start/run” an appliance: poner el lavavajillas = run the dishwasher. You can hear poner las tazas en el lavavajillas, but meter is more precise for loading.

Why is it en el lavavajillas and not al lavavajillas?

With meter, the standard preposition for “into” is en (or dentro de): meter algo en el lavavajillas. Using a/al here is nonstandard in Spain. You can also say meterlo dentro del lavavajillas.

Is lavavajillas masculine even though it ends in -s? What about the plural?

Yes, it’s masculine: el lavavajillas. It’s invariable in form: singular and plural are both lavavajillas.

Singular: un lavavajillas nuevo
Plural: dos lavavajillas nuevos

Can lavavajillas mean the detergent too?

Yes, in Spain lavavajillas can be the machine or the detergent. Context disambiguates:

Machine: meter las tazas en el lavavajillas
Detergent: echar lavavajillas en el compartimento / detergente para el lavavajillas

Can I replace las tazas with a pronoun?

Yes. Use the direct object pronoun las for tazas.

Simple: …sino que las mete en el lavavajillas.
Progressive (two options): …sino que las está metiendo… / …sino que está metiéndolas… (note the accent in metiéndolas).

Can I move ahora around?

Yes; the meaning is similar, with slight changes in emphasis:

Ahora mi hermana no friega, sino que… (fronted “now” sets the time frame)
Mi hermana ahora no friega, sino que…
Mi hermana no friega ahora, sino que… (time adverb at the end)

Do I need the comma before sino que?

Recommended, yes. When sino (que) connects full clauses, Spanish typically uses a comma before it:

No friega ahora, sino que mete…

Can I drop the que and say sino mete?

Not when the second part has a conjugated verb. With a finite verb you need sino que. Without que you’d have to change the structure:

Correct: No friega ahora, sino que mete…
Alternative with an infinitive: En vez de fregar ahora, mete…

How does this compare to using en vez de or en lugar de?

sino que must follow a negation and presents a correction: No friega…, sino que mete…
en vez de/en lugar de + infinitive/noun expresses substitution without needing a negation: En vez de fregar, mete las tazas…

What’s the difference between taza, vaso, and copa?

taza: cup with a handle (coffee/tea, can be ceramic).
vaso: tumbler/glass (no stem, usually for water/juice).
copa: stemmed glass (wine, champagne).

Why isn’t que accented in sino que?

Because it’s the conjunction que. The accented qué is for questions/exclamations. Here it just links clauses: sino que.

Is there a verb specifically for “to load/unload the dishwasher”?

Yes:

cargar el lavavajillas = to load the dishwasher
vaciar or descargar el lavavajillas = to unload/empty it Your sentence uses meter to express loading, which is also common.