Learn Hardcore Spanish (Latin America): Mis botas fueron limpiadas por mi hermana anoche. - My boots were cleaned by my sister last night.

Conversational active: Mi hermana me limpió las botas anoche.
Se-passive without agent: Se limpiaron las botas anoche. (no mention of who did it) Use the ser passive (as in the original) when you want to highlight the boots or include the agent explicitly in a formal tone.

How else could I say it more naturally in conversation?

Two very common options:

Mi hermana me limpió las botas anoche. (most typical; the indirect object me shows the owner)
Mi hermana limpió mis botas anoche. (also fine; uses the possessive)

Why does limpiadas end in -as? Does the participle have to agree?

Yes. In the ser passive, the past participle agrees with the subject in gender and number:

Mis botas fueron limpiadas… (feminine plural)
Mi sombrero fue limpiado… (masculine singular)
Las casas fueron pintadas… (feminine plural)

But with haber I’ve seen no agreement. Why?

With haber (perfect tenses), the participle is invariable:

Han limpiado mis botas. (no agreement on limpiado) However, if you use haber + ser to form a passive perfect, the participle after ser still agrees:
Mis botas han sido limpiadas.

Can I say fueron limpiado without the -s/-a agreement?

No. In the ser passive, agreement is mandatory. It must be fueron limpiadas to match botas (feminine plural).

Why is it por mi hermana and not para mi hermana or de mi hermana?

In passive constructions, the agent (the doer) is introduced with por: fueron limpiadas por mi hermana.

para expresses purpose or destination.
de can mark possession or origin, not the agent in a passive.

Can I use the se-passive and still add the agent, like Se limpiaron las botas por mi hermana?

No. The se passive normally does not take an explicit agent. If you need to mention the agent, use the ser passive or switch to the active voice.

Could I use estar instead of ser, like Mis botas estuvieron limpiadas?

Use ser for the passive action: fueron limpiadas.
Use estar to describe the resulting state: Mis botas estaban limpias anoche (they were clean). Estuvieron limpiadas is unnatural.

Is the tense right with anoche? Could I say han sido limpiadas?

With anoche, Latin American Spanish strongly prefers the preterite: fueron limpiadas. Han sido limpiadas is grammatical but sounds more Peninsular/European and is odd with anoche in much of Latin America.

Where can I place anoche?

Common placements:

Mis botas fueron limpiadas por mi hermana anoche.
Anoche, mis botas fueron limpiadas por mi hermana. Latin American alternatives to anoche include ayer por la noche and, in many regions, ayer en la noche.

Should I say mis botas or las botas? I’ve heard Spanish avoids possessives with body parts/clothes.

Both are possible, but patterns differ:

With an indirect object pronoun, Spanish favors the article: Mi hermana me limpió las botas.
In the passive with ser, there’s no easy place for me, so mis botas is normal: Mis botas fueron limpiadas…

Can I change the word order around por mi hermana?

Yes, but keep it natural:

Neutral: Mis botas fueron limpiadas por mi hermana anoche.
To emphasize the agent: Por mi hermana fueron limpiadas mis botas anoche. (formal/emphatic) Avoid awkward splits like inserting por mi hermana mid-verb phrase.

Are there better verbs for boots than limpiar?

For footwear, you’ll often hear:

Lustrar (to shine/polish leather): Mis botas fueron lustradas…
Pulir (to polish)
Encerar (to wax) Limpiar is general and always correct; the others add nuance.

Any accent or spelling pitfalls in this sentence?

mi (my) has no accent; mí (me) takes an accent only after prepositions, e.g., para mí. Here it’s mi hermana.
anoche has no accent.
fueron has no accent.
Everything in the sentence is spelled correctly.