Хоть сообщение и было коротким, моя сестра всё равно исправила одну заглавную букву и одну строчную.

It is a concessive pattern: it introduces an idea like although / even though.

So:

• Хоть сообщение и было коротким = Although the message was short

A few useful notes:

• хоть here is a common, more conversational alternative to хотя
• the и in this pattern does not mean normal and
• together, хоть ... и ... works as a set

So the sentence structure is basically:

• Even though X, Y still happened

That fits nicely with всё равно later in the sentence.

Right — here и is part of the concessive construction хоть ... и ....

In this pattern, и often appears before the predicate or the part being highlighted:

• Хоть сообщение и было коротким...

It does not add a separate meaning like and. It just helps mark the clause as although/even though.

You can think of it as a structural particle in this pattern rather than a normal conjunction.

Because сообщение is a neuter singular noun.

In the past tense, Russian verbs agree with the subject in gender and number:

• был = masculine
• была = feminine
• было = neuter
• были = plural

So:

• сообщение было = the message was

If the subject were письмо (also neuter), you would still use было. If it were текст (masculine), you would use был.

Because after быть in the past tense, Russian very often uses the instrumental case for a predicate adjective.

So:

• сообщение было коротким

Here коротким is the instrumental singular form of короткий.

This is a very common pattern:

• он был счастливым
• она была уставшей
• письмо было коротким

For an English speaker, it can feel strange because English just uses a plain adjective after was. Russian often uses the instrumental in this kind of structure.

Also, короткое would be nominative neuter singular, but that would not sound natural here in standard modern Russian.

Here всё равно means still, all the same, or anyway.

So the idea is:

• Even though the message was short, my sister still corrected...

This phrase is very common in Russian, but it has more than one use. For example:

1. still / all the same

   • Я устал, но всё равно пойду.
   • I’m tired, but I’ll go anyway.

2. it makes no difference

   • Мне всё равно.
   • I don’t care / It’s all the same to me.

In your sentence, it clearly has the first meaning.

Because the sentence describes a completed action with a result.

• исправила = corrected / fixed as a finished act

That fits the context: she corrected one uppercase letter and one lowercase one, and the action is presented as done.

Compare:

• исправила = completed the correction
• исправляла = was correcting / used to correct / corrected repeatedly

So if you said:

• моя сестра исправляла буквы

that could suggest a process, repetition, or background action.

But here the sentence is about the fact that she did make those corrections, so perfective исправила is the natural choice.

Because буква is a feminine noun, and it is the direct object of исправила, so it goes into the accusative case.

Forms:

• одна → одну
• заглавная → заглавную
• буква → букву

So:

• одну заглавную букву

This literally matches the grammar of corrected one capital letter.

Because Russian often omits a repeated noun when it is already obvious from the context.

So:

• одну заглавную букву и одну строчную means
• one uppercase letter and one lowercase letter

The full version would be:

• одну заглавную букву и одну строчную букву

But repeating букву is unnecessary, so it is left out.

In grammar terms, строчную is an adjective being used on its own because the noun is understood.

This is very natural in Russian.

They describe types of letters:

• заглавная буква = capital letter / uppercase letter
• строчная буква = lowercase letter

A useful extra note:

• заглавная буква is the standard way to say capital letter
• you may also see прописная буква, which can also mean capital letter

So learners should recognize both, but заглавная and строчная are a very natural pair.

Because the first part is a subordinate concessive clause:

• Хоть сообщение и было коротким, ...

Russian separates that clause from the main clause with a comma, just as English often does with Although the message was short, ...

So the sentence is divided into:

1. Хоть сообщение и было коротким
2. моя сестра всё равно исправила одну заглавную букву и одну строчную

No, Russian word order is fairly flexible. The version in the sentence is natural, but other orders are possible.

Your sentence:

• Хоть сообщение и было коротким, моя сестра всё равно исправила...

This sounds natural and balanced.

You could also move всё равно for emphasis:

• Хоть сообщение и было коротким, всё равно моя сестра исправила...
• Моя сестра всё равно исправила...

The basic meaning stays similar, but the emphasis shifts slightly.

In the original sentence, всё равно sits in a very common position and links nicely to the concessive idea: even though..., she still...

Yes.

You could say:

• Хотя сообщение и было коротким, моя сестра всё равно исправила...

That would mean the same thing.

The difference is mostly stylistic:

• хотя = more neutral / standard
• хоть = often a bit more conversational

So in this sentence, хоть sounds perfectly natural, just slightly less formal than хотя.