Learn Hardcore German: Meine Schwester friert nie, ihr ist fast immer warm. - My sister never feels cold; she is almost always warm.

sie ist warm – literally: she is warm (to the touch / has a warm personality / is sexually aroused)
ihr ist warm – literally: to her it is warm → natural way to say she feels warm / she is warm (not cold)

For talking about how hot/cold someone feels, German usually uses a dative pronoun (mir / dir / ihr / ihm / uns / euch / ihnen) + ist kalt / warm:

Mir ist kalt. – I am (feeling) cold.
Ihr ist warm. – She is (feeling) warm.

So ihr ist fast immer warm is the idiomatic way to say she almost always feels warm.

What exactly does friert mean, and how is it different from ihr ist kalt?

frieren is a verb that means to feel cold / to freeze (in the bodily sense).

Meine Schwester friert nie. – My sister never feels cold / never freezes.
Ihr ist nie kalt. – She is never cold (she never feels cold).

In many contexts, frieren and jemandem ist kalt overlap in meaning:

Ich friere. ≈ Mir ist kalt.

Small nuance:

frieren is more about the process or state of freezing/feeling cold.
jemandem ist kalt states that someone is cold right now.

In the given sentence, friert nie and ihr ist fast immer warm nicely mirror each other: she doesn’t feel cold; she almost always feels warm.

Why is the pronoun ihr used, and what case is it in?

Here ihr is dative singular feminine and refers back to meine Schwester.

Pattern:
jemandem (dative) ist kalt / warm / heiß / schlecht / langweilig, etc.

Examples:

Meinem Bruder ist kalt. – My brother is cold.
Ihr ist warm. – She is warm / She feels warm.
Uns ist langweilig. – We are bored.

So:

Nominative: sie (she)
Dative: ihr (to her)

In ihr ist fast immer warm, ihr is not the subject in the nominative sense; it’s a dative experiencer: to her it is warm.

Is the sentence Meine Schwester friert nie, sie ist fast immer warm also correct?

Grammatically, yes:
Meine Schwester friert nie, sie ist fast immer warm is possible.

But it sounds odd or ambiguous in everyday German because sie ist warm is usually understood as:

she has a warm personality, or
she is warm to the touch, or, in some contexts,
she is turned on (colloquial, potentially sexual).

To express she almost always feels warm (not cold), speakers strongly prefer:

Ihr ist fast immer warm.

So your original sentence is both more natural and less ambiguous.

Why is nie placed right after friert?

German places most simple adverbs (like nie, immer, oft) after the conjugated verb in a normal main clause:

Sie arbeitet immer. – She always works.
Er kommt nie. – He never comes.
Meine Schwester friert nie. – My sister never feels cold.

You could also say Meine Schwester friert fast nie if you want to say almost never. But the basic position of nie after the verb is standard.

What does fast immer mean exactly, and why is fast before immer?

fast immer means almost always.

immer = always
fast = almost / nearly

When fast modifies another adverb, it usually comes in front of it:

fast nie – almost never
fast immer – almost always
fast überall – almost everywhere

So ihr ist fast immer warm = she feels warm almost all the time.

Why is warm not declined (why not warme or something similar)?

In ihr ist fast immer warm, warm is a predicative adjective (used after sein / ist) and it does not take an ending in German:

Es ist warm. – It is warm.
Mir ist warm. – I am warm / I feel warm.
Deiner Schwester ist warm. – Your sister is warm.

Adjectives only take endings when they come directly before a noun:

eine warme Jacke – a warm jacket
warme Hände – warm hands

Here, warm describes a state, so it stays in its basic form.

Is frieren a regular verb, and how is it conjugated?

frieren is slightly irregular, because the stem vowel can change in the past tense, but in the present tense it is regular in its endings:

Present tense:

ich friere
du frierst
er / sie / es friert
wir frieren
ihr friert
sie frieren

In your sentence:

Meine Schwester friert nie → sie = 3rd person singular, so friert.

Past forms (for reference):

Präteritum: ich fror – I was freezing / I froze
Partizip II: gefroren – frozen

Why do we have a comma between the two parts of the sentence?

The sentence has two main clauses:

Meine Schwester friert nie – My sister never feels cold.
ihr ist fast immer warm – she almost always feels warm.

In German, when two independent main clauses are simply placed next to each other, they’re usually separated by a comma:

Es regnet, ich bleibe zu Hause.
Meine Schwester friert nie, ihr ist fast immer warm.

You could also join them with a coordinating conjunction:

Meine Schwester friert nie, denn ihr ist fast immer warm. – My sister never feels cold, because she almost always feels warm.

Could I say Meiner Schwester ist fast immer warm instead of Ihr ist fast immer warm?

Yes, that’s correct as well.

Meiner Schwester ist fast immer warm.
Ihr ist fast immer warm.

Both mean the same: My sister almost always feels warm.

Difference:

Meiner Schwester (dative with a full noun phrase) is more explicit.
Ihr is the dative pronoun referring back to meine Schwester already mentioned.

Your original structure is very natural: mention the full noun once, then use the pronoun:

Meine Schwester friert nie, ihr ist fast immer warm.

Why is the verb ist second in ihr ist fast immer warm? Isn’t the subject missing?

German main clauses follow a V2 rule: the finite verb (here ist) must be in second position.

In ihr ist fast immer warm:

1st position: ihr (dative experiencer)
2nd position: ist (finite verb)
Rest: fast immer warm

There is no separate nominative subject word like es in this sentence; the structure is more like:

(Es) ist ihr warm. → To her it is warm.

In everyday German, the es is often omitted when you have [dative + ist + adjective]:

(Es) ist mir kalt. → Mir ist kalt.
(Es) ist ihr warm. → Ihr ist warm.

The verb still stays in second position relative to the first element (ihr).