Моя младшая сестра может читать о кошках сколько угодно и совсем не устаёт.

Because сестра is a feminine singular noun, so the words that describe it must also be feminine singular.

• моя = my (feminine)
• младшая = younger (feminine)
• сестра = sister (feminine)

Compare:

• мой младший брат = my younger brother
• моя младшая сестра = my younger sister

Russian adjectives and possessive words must agree with the noun in gender, number, and case.

Usually it means younger sister.

In family relationships, Russian often uses старший / младший to mean older / younger, not necessarily oldest / youngest.

So:

• старшая сестра = older sister
• младшая сестра = younger sister

If someone has more than two sisters, младшая сестра still usually just means a sister who is younger than the speaker, not automatically the youngest of all.

Because Russian often omits subject pronouns when the subject is already clear.

Here, моя младшая сестра is the subject, so once that has been said, Russian does not need to repeat она.

Literal structure:

• Моя младшая сестра может читать... и совсем не устаёт.
• My younger sister can read... and does not get tired at all.

In English, repeating the subject or using she is often necessary. In Russian, it often is not.

Because after мочь (can / be able to), Russian uses the infinitive.

So:

• может читать = can read
• может писать = can write
• может говорить = can speak

Not:

• может читает ❌

This is similar to English:

• can read
• not can reads

Here может most naturally expresses ability or capacity.

So the sentence suggests something like:

• she is able to read about cats for as long as she wants
• she can keep reading about cats without getting tired

It does not sound mainly like permission (she is allowed to read), and it does not mainly mean mere possibility (it may happen that she reads).

Context matters, but in this sentence it feels like capacity/endurance.

Читать is imperfective because the sentence talks about an ongoing, repeated, or unrestricted activity, not one completed act.

Here the idea is:

• she can read and read
• she can read for as long as she likes
• the focus is on the process, not finishing one text

That matches imperfective very well.

If you used прочитать, the meaning would shift toward reading something through / finishing reading:

• может прочитать = can read through, can finish reading

That does not fit as naturally with сколько угодно, which emphasizes unlimited duration or amount.

О кошках means about cats.

The preposition о / об usually takes the prepositional case.

• кошка = cat
• prepositional singular: о кошке = about a cat
• prepositional plural: о кошках = about cats

So:

• читать о кошках = to read about cats

This is a very common pattern:

• говорить о книге = talk about a book
• думать о работе = think about work
• читать о животных = read about animals

Сколько угодно is an idiomatic expression meaning something like:

• as much as one likes
• as long as one wants
• any amount
• to one’s heart’s content

In this sentence, because the verb is читать (to read), it can mean both:

• she can read as much as she wants
• she can read for as long as she wants

Russian often leaves that broad and natural.

Some examples:

• Ешь сколько угодно. = Eat as much as you want.
• Жди сколько угодно. = Wait as long as you want.
• Бери сколько угодно. = Take as much as you want.

Because here сколько угодно functions as a set expression closely tied to the verb читать, not as a full subordinate clause with its own grammatical structure.

So:

• может читать сколько угодно = can read as much/as long as she wants

It behaves almost like an adverbial phrase.

Learners sometimes expect a comma because English often separates clauses differently, but in Russian this phrase is commonly written without a comma in this kind of sentence.

In negative sentences, совсем не often means:

• not at all
• absolutely not

So:

• устаёт = gets tired
• не устаёт = does not get tired
• совсем не устаёт = does not get tired at all

Here совсем strengthens the negation.

Compare:

• Я не устал. = I’m not tired.
• Я совсем не устал. = I’m not tired at all.

Because Russian often uses the verb уставать / устать to express getting tired or becoming tired, not just the state of already being tired.

• уставать = to get tired, to become tired (imperfective)
• устать = to get tired, to become tired (perfective)

So:

• совсем не устаёт = she does not get tired at all

If you said something with быть усталой, that would describe a state:

• Она усталая. = She is tired.
• Она бывает усталой. = She is sometimes tired.

But this sentence is about what happens while she reads: she doesn’t become tired.

The imperfective present can describe:

• what is happening now
• what happens regularly
• a general characteristic

Here it most naturally expresses a general characteristic:

• she can read about cats as much as she wants and doesn’t get tired

So the meaning is not just about one specific moment. It suggests that this is typical of her.

Yes, Russian word order is flexible, and different orders can change the emphasis.

The given sentence is very natural:

• может читать о кошках сколько угодно

This presents the idea smoothly: read about cats for as long/as much as she wants.

Other orders are possible, for example:

• может сколько угодно читать о кошках
• о кошках может читать сколько угодно

These sound a bit different in emphasis:

• сколько угодно earlier can emphasize the unlimited amount/duration
• о кошках earlier can emphasize the topic cats

The original word order is neutral and natural.

Yes, it can often be written as устает in ordinary Russian spelling, because ё is frequently replaced by е in printed text.

So both may appear:

• устаёт
• устает

But for learners, ё is very helpful because it shows the correct pronunciation and stress:

• устаёт = stress on the last syllable

So even though Russians often omit the dots, it is useful to remember that the actual sound here is ё.

You could translate it literally in pieces, but that would sound unnatural in English.

A more natural breakdown is:

• Моя младшая сестра = my younger sister
• может читать = can read
• о кошках = about cats
• сколько угодно = as much / as long as she wants
• и совсем не устаёт = and doesn’t get tired at all

Natural English translations would be:

• My younger sister can read about cats for as long as she wants and never gets tired at all.
• My younger sister can read about cats as much as she likes and doesn’t get tired at all.

So yes, a word-for-word translation is possible for study, but the idiomatic English version is better for understanding the whole sentence naturally.