imouto ha karai mono ga tokui desu ga, watasi ha amari taberaremasen.

Because 辛い has more than one reading and meaning.

• からい = spicy / hot in taste
• つらい = painful / hard / difficult emotionally

In this sentence, it is describing food, so からい is the natural reading:

• 辛い物 = spicy things / spicy food

A learner should get used to the fact that kanji often have multiple readings, and the correct one depends on context.

Yes, 物（もの） literally means thing, but in this kind of sentence it often means things of that kind, and with 辛い it naturally refers to spicy foods / spicy things to eat.

So:

• 辛い物 = spicy things
• in natural English: spicy food

Japanese often uses 物 in a broader way than English would use thing. It does not sound strange in Japanese.

This is a very common question about particles.

• 妹は: は marks 妹 as the topic
• 私は: は marks 私 as the contrasting topic in the second clause
• 辛い物が: が marks the thing connected to 得意

The pattern with 得意 is usually:

• X は Y が 得意です
• literally: As for X, Y is something they are good at

So here:

• 妹は 辛い物が 得意です
• literally: As for my younger sister, spicy things are her strong point

In more natural English, that becomes something like My younger sister is good with spicy food or can handle spicy food well.

That is exactly what makes this sentence a little interesting.

得意 usually means good at, one’s strong point, or something one handles well. It is often used for skills:

• 英語が得意です = I’m good at English
• 料理が得意です = I’m good at cooking

But it can also be used with things like food to mean:

• good with
• able to handle
• not bothered by

So:

• 辛い物が得意です does not mean the sister is skilled at creating spicy food
• it means she handles spicy food well / is fine with spicy food

This is a natural Japanese usage.

That が is not the subject particle. It is the conjunction but.

So the sentence has two parts:

• 妹は辛い物が得意ですが、
• 私はあまり食べられません。

This が connects the two ideas and shows contrast:

• My younger sister is good with spicy food, but...
• I can’t eat much of it.

In polite Japanese, ですが is very common as a softer, more formal way to connect clauses.

あまり is commonly used with a negative verb to mean:

• not very
• not much
• not really

So:

• あまり食べられません = I can’t eat very much
• not simply I can’t eat at all

This is an important pattern:

• あまり + negative

Examples:

• あまり分かりません = I don’t understand very well
• あまり行きません = I don’t go very often

Without a negative ending, あまり usually does not work the same way in standard modern Japanese.

Because 食べられません is the potential form, meaning:

• cannot eat
• am not able to eat

That gives the sentence the nuance of being unable to handle spicy food, rather than simply choosing not to eat it.

Compare:

• 食べません = I do not eat it / I won’t eat it
• 食べられません = I cannot eat it

Here the speaker is saying they are not able to eat much spicy food, probably because it is too spicy for them.

Yes. Japanese often omits information that is obvious from context.

In the second clause, the speaker does not repeat 辛い物. It is understood:

• 私は（辛い物は）あまり食べられません。

So the full idea is:

• As for me, I can’t eat much spicy food.

Japanese avoids repetition much more freely than English does.

Using は with 私 creates a clear contrast with 妹.

The structure is:

• 妹は ... ですが、私は ...

This is like saying:

• As for my younger sister..., but as for me...

So は helps set up a contrast between the two people:

• the sister handles spicy food well
• the speaker does not

This kind of paired contrast with は is extremely common in Japanese.

Yes, it could.

• 辛い物 = spicy things / spicy food
• 辛い食べ物 = spicy foods

Both are understandable. However, 辛い物 is very natural and common in everyday Japanese. It sounds lighter and less wordy.

Japanese often prefers the shorter expression when the meaning is already clear.

Yes. It is in polite style because it uses:

• です
• ません
• ですが

So this is appropriate in normal polite conversation, writing, or when speaking to someone you are not especially close to.

A more casual version might be:

• 妹は辛い物が得意だけど、私はあまり食べられない。

Same basic meaning, but less formal.