Mi hermana se resbaló en las escaleras y ahora tiene dolor en la rodilla y en el codo.

In Spanish, resbalarse is very commonly used to mean to slip. The se is part of the verb here, not a separate word meaning herself in the English sense.

So:

• resbalar = to slip / to slide
• resbalarse = to slip

In everyday Spanish, se resbaló sounds very natural for she slipped.

In this sentence, Mi hermana se resbaló means My sister slipped.

It is the preterite tense, used for a completed action in the past.

• se resbaló = she slipped

The sentence talks about one finished event: she slipped on the stairs. That is why the preterite is used.

The accent mark shows where the stress goes: res-ba-LÓ.

It also helps distinguish the preterite form:

• resbalo = I slip
• resbaló = he/she slipped

So the accent is important both for pronunciation and meaning.

En las escaleras means on the stairs or on the staircase.

Spanish often uses en where English might use on or in, depending on the context. So even though English says on the stairs, Spanish says en las escaleras.

Spanish often uses the definite article more than English does.

So en las escaleras literally means on the stairs, and that sounds natural in Spanish. Leaving out las here would usually sound incomplete.

Spanish often uses tener with nouns in situations where English uses be or another structure.

So:

• tiene dolor = literally has pain
• natural English meaning = is in pain / has pain

This is a normal Spanish pattern.

Spanish often uses the definite article with body parts instead of possessives when it is already clear whose body parts they are.

So Spanish prefers:

• le duele la cabeza
• tiene dolor en la rodilla
• se rompió el brazo

rather than constantly saying su cabeza, su rodilla, su brazo.

Here, since the sentence is already about my sister, it is obvious that the knee and elbow are hers.

The repetition makes the sentence clearer and more natural.

You may also hear:

• dolor en la rodilla y el codo

but en la rodilla y en el codo is very clear and nicely balanced. It shows that the pain is in both places.

Because nouns in Spanish have grammatical gender:

• la rodilla = the knee
• el codo = the elbow

This is something you simply have to learn with each noun. The gender does not always match any logical rule from English.

Yes, absolutely. That is another very common way to express this idea.

For example:

• Mi hermana se resbaló en las escaleras y ahora le duele la rodilla y el codo.

That means My sister slipped on the stairs and now her knee and elbow hurt or she has pain in her knee and elbow.

Both versions are correct, but they focus slightly differently:

• tiene dolor en... = she has pain in...
• le duele(n)... = ...hurts / hurt her

In Spanish, the simple present is often used where English might use a more progressive idea.

• ahora tiene dolor = now she has pain

Using a progressive form like está teniendo would usually sound strange here. Spanish does not use the continuous form as often as English does.

Usually here it is best understood as pain, not a pain.

• tiene dolor en la rodilla = she has pain in her knee

If you say un dolor, that often refers to a pain in a more countable or specific sense, but in this sentence dolor works naturally as a general, uncountable idea.

The structure is very straightforward:

• Mi hermana = subject
• se resbaló = verb
• en las escaleras = where it happened
• y ahora = and now
• tiene dolor = has pain
• en la rodilla y en el codo = where the pain is

So the sentence follows a common Spanish order: subject + verb + extra details.

Yes, Spanish could leave it out if the context is already clear:

• Se resbaló en las escaleras y ahora tiene dolor en la rodilla y en el codo.

Because the verb forms already show she, Spanish often drops subject pronouns and sometimes even noun subjects if they are understood.

But including Mi hermana is perfectly natural when you want to make clear who you are talking about.

Not exactly.

• resbalarse = to slip
• caerse = to fall

Sometimes slipping causes a fall, but they are not the same thing. In this sentence, se resbaló means she slipped. It does not necessarily say whether she fully fell down, though that may be implied by the context.

The sentence is already natural. But in casual speech in Spain, people might also say something like:

• Mi hermana se cayó en las escaleras y ahora le duele la rodilla y el codo.
• Mi hermana se resbaló en las escaleras y ahora le duelen la rodilla y el codo.

These are just alternative natural ways of saying something very similar. Your original sentence is completely correct and normal.