Learn Hardcore French: Son frère est malin aussi, par contre il parle trop vite. - Her brother is clever too; however, he speaks too fast.

Both are possible. Son agrees with the noun possessed, not with the owner. Because frère is masculine singular, you use son whether the owner is male or female. If you need to make the owner explicit, say le frère de Marie (Marie’s brother) or, in speech, son frère à Marie / son frère à elle. For their brother, use leur frère. Note: with a feminine noun starting with a vowel, you also use son for euphony (e.g., son amie, not sa amie).

What nuance does malin have compared with intelligent?

Malin often implies cleverness with a hint of craftiness, street smarts, or slyness; the tone can be admiring or mildly critical depending on context. Intelligent is a neutral way to say smart. Near-synonyms: astucieux, rusé, futé, débrouillard (each with its own nuance).

What is the feminine of malin, and how does agreement work?

The feminine is irregular: maligne (not maline). Examples:

Masculine singular: Il est malin.
Feminine singular: Elle est maligne.
Masculine plural: Ils sont malins.
Feminine plural: Elles sont malignes.

Is placing aussi at the end correct? What’s the difference between Il est malin aussi and Il est aussi malin?

Il est malin aussi = He is clever too/also. This is the clearest way to express also/too.
Il est aussi malin tends to mean as clever (…as someone else) and usually calls for que: Il est aussi malin que son frère. Without que, it can sound incomplete or ambiguous in careful French.

What does par contre mean exactly, and how formal is it?

It means on the other hand/however and introduces a contrast. It’s very common in speech and neutral in everyday writing. In more formal writing, many prefer en revanche or cependant.

Can I replace par contre with mais or en revanche? Any nuance differences?

Mais = but, the most general and neutral contrastor.
Par contre = however/on the other hand; very common in conversation.
En revanche = on the other hand, often a bit more formal or written.
Cependant / toutefois = however/nevertheless; more formal. All can work here; choose by register and tone.

Is the comma placement around par contre okay?

Yes. You can write:

…, par contre, il parle trop vite. (commas on both sides, treating it parenthetically)
Or split into two sentences: … . Par contre, il parle trop vite. Some writers use only one comma before par contre, but bracketing it or starting a new sentence often reads more clearly.

Why do we repeat il? Can I omit the subject pronoun after the comma?

You must keep it. French requires an explicit subject pronoun: …, par contre, il parle… Dropping il (…, par contre, parle…) is ungrammatical in standard French.

What’s the difference between trop vite and très vite?

Trop vite = too fast (excessive, problematic).
Très vite = very fast (not necessarily a problem). In casual speech, trop can be used as an intensifier meaning very (e.g., trop bien), but in il parle trop vite, it keeps the literal excessive meaning.

Could I use rapidement instead of vite?

Yes: Il parle trop rapidement is correct, though vite is more common and natural in everyday speech. Note that vitement does not exist.

Can I start a sentence with Par contre,?

Yes. Par contre, il parle trop vite. is perfectly natural. In formal writing you might prefer En revanche, il… or Cependant, il….

Any pronunciation tips for this sentence?

frère has an open ê sound (the grave accent matters).
malin ends with a nasal vowel; the final n isn’t pronounced as an n.
trop ends with a silent p before a consonant (so in trop vite, the p is silent).
parle ends with a silent e.
contre has a nasal on on (similar to the vowel in French bon), and a French r.

If I wanted to say he also speaks too fast (not contrasting, just adding), how would I phrase it?

Use aussi to focus on the subject: Lui aussi, il parle trop vite. You can also say Il parle lui aussi trop vite. Avoid Il parle aussi trop vite if there’s any risk of ambiguity about what aussi is modifying.

Can I say Son frère aussi est malin? How does that differ from Son frère est malin aussi?

Both are possible but the emphasis shifts:

Son frère aussi est malin highlights the subject (the brother is also among those who are clever).
Son frère est malin aussi states the property and adds also/too more neutrally.