Mi hermana dice que debería dormir más, pero siempre se acuesta tarde.

In Spanish, when a verb like decir (to say) is followed by a whole sentence (a clause), you normally need the conjunction que to introduce that clause.

• Mi hermana dice que debería dormir más.
  = My sister says (that) I/she should sleep more.

In English, that can be dropped: My sister says I should sleep more.
In Spanish, que usually cannot be dropped in this structure; Mi hermana dice debería dormir más sounds wrong.

Grammatically, debería is ambiguous by itself because Spanish often drops subject pronouns:

• yo debería (I should)
• ella debería (she should)

In Mi hermana dice que debería dormir más, pero siempre se acuesta tarde, the second part:

• pero siempre se acuesta tarde = but she always goes to bed late

clearly refers to my sister (3rd person singular: se acuesta).
So the most natural interpretation is:

• My sister says she should sleep more, but she always goes to bed late.

If you wanted to make it 100% explicit, you could say:

• Mi hermana dice que ella debería dormir más… (she should)
• Mi hermana dice que yo debería dormir más… (I should)

Debería is the conditional form of deber (to have to / should), 3rd person singular (or 1st person singular — same form):

• yo debería = I should / I ought to
• él / ella debería = he / she should / ought to

The conditional of deber is commonly used for:

• soft advice / recommendation
  
  • Deberías estudiar más. = You should study more.
• milder, less forceful obligation than debe or tiene que.

So debería dormir más = should sleep more (advice, not a strong order).

When deber (or debería) is used in the sense of should / ought to, it is followed by an infinitive:

• deber + infinitive
  • Debo dormir. = I must / should sleep.
  • Deberías comer. = You should eat.
  • Debería dormir más. = (He/She/I) should sleep more.

So dormir stays in the infinitive, not conjugated to duermo.

In Spanish, más (more) usually goes:

• before adjectives/adverbs:
  
  • más rápido = faster
  • más cansado = more tired
• after verbs when it means “more [of this action]”:
  
  • trabajar más = to work more
  • comer más = to eat more
  • dormir más = to sleep more

So dormir más is the natural word order: verb + más.
más dormir would sound odd or need a special context to make sense.

Acostarse is a reflexive verb, meaning literally “to put oneself to bed / to lie oneself down”, and in practice it means “to go to bed”.

• Infinitive: acostarse
• Reflexive pronouns: me, te, se, nos, se

So:

• Yo me acuesto. = I go to bed.
• Ella se acuesta. = She goes to bed.

Here, se is the reflexive pronoun for ella (or él / usted), required by the verb.

Without the reflexive se, acostar means “to put someone else to bed”:

• Yo acuesto a mi hijo. = I put my son to bed.

With se, acostarse means “to go to bed (oneself)”:

• Mi hermana se acuesta tarde. = My sister goes to bed late.

Since your sister is going to bed herself, you must use the reflexive form: se acuesta.

Acostar is a stem‑changing verb (o → ue) in the present tense for most forms:

• yo acuesto
• tú acuestas
• él / ella / usted acuesta
• ellos / ellas / ustedes acuestan

But:

• nosotros / nosotras acostamos (no change)
• vosotros / vosotras acostáis (no change — mostly in Spain)

So in se acuesta, the root vowel o changes to ue as part of regular present‑tense conjugation: acostar → acuesta.

Spanish often omits subject pronouns because the verb ending already shows the subject:

• se acuesta = clearly he/she/it/usted goes to bed

So:

• Ella siempre se acuesta tarde.
• Siempre se acuesta tarde.

Both mean the same; the second just drops ella because it’s understood from context and the verb form.

In this sentence, tarde is an adverb meaning late:

• Se acuesta tarde. = She goes to bed late.

When talking about the time of day, la tarde (with article, usually) is the afternoon:

• Por la tarde = in the afternoon.

Here there is no article and it’s used as an adverb, so it means late, not “afternoon”.

No. When decir is followed by a full clause, Spanish normally requires que:

• correct: Mi hermana dice que debería dormir más.
• incorrect / very unnatural: Mi hermana dice debería dormir más.

Spanish doesn’t allow dropping que in this structure the way English can drop that.

• Mi hermana dice que debería dormir más.

  • My sister says (that) [someone] should sleep more.
  • General statement; no direct object. Context decides who “should sleep more”.

• Mi hermana me dice que debería dormir más.

  • My sister tells me that I should sleep more.
  • me = to me; it makes it clear she is talking to me, and usually implies I am the one who should sleep more.

Adding me turns it into “she tells me” instead of just “she says”.

Yes, but the nuance changes:

• Mi hermana dice que debe dormir más.

  • My sister says she must / has to sleep more.
  • Sounds stronger, more like an obligation.

• Mi hermana dice que tiene que dormir más.

  • My sister says she has to sleep more.
  • Also a fairly strong obligation or necessity.

• Mi hermana dice que debería dormir más.

  • My sister says she should sleep more.
  • Softer, more like advice or a recommendation.

So debería is the most “polite/advisory” of the three.

Spanish punctuation normally places a comma before “pero” when it introduces a contrasting clause, similar to English:

• Mi hermana dice que debería dormir más, pero siempre se acuesta tarde.

First clause: what she says (she should sleep more).
Second clause: the contrast (she actually goes to bed late).
The comma marks this contrast clearly.