Learn Hardcore Japanese: zisin no taisaku wo kazoku to issyo ni kangaemasu. - I think about countermeasures for earthquakes together with my family.

The particle の is used to connect two nouns where the first one modifies the second. Here, 地震の対策 literally means “countermeasures of (or for) earthquakes,” i.e. “earthquake countermeasures.”

What role does を play after 対策?

を marks 対策 as the direct object of the verb 考えます. So the sentence literally says “(I) think (about) earthquake countermeasures.”

Why is と used in 家族と instead of another particle?

The particle と means “with” when talking about doing something together. 家族と means “with (my) family.”

What about 一緒に? Why not just 一緒?

一緒 is a noun or na-adjective, so you need に to make it an adverb modifying the verb. 一緒に考えます means “think together.” Without に, it wouldn’t link properly to the verb.

Could the word order change, for example putting 一緒に earlier?

Japanese is relatively flexible, but the verb almost always comes last. You could say 家族と一緒に地震の対策を考えます, and it still means the same. The particles keep each phrase clear, so you can shuffle noun phrases before the verb.

Why is the verb 考えます in this polite form? Could I use the dictionary form?

考えます is the polite non-past form (masu-form) of 考える. In casual speech or writing you could use 考える: 地震の対策を家族と一緒に考える. Choose 〜ます or plain form depending on your level of formality.

Is there any nuance difference between 対策を考える and, say, 準備をする?

Yes. 対策を考える focuses on “coming up with or planning measures,” while 準備をする (to make preparations) emphasizes the act of getting things ready. Both are related, but 対策を考える is more about the planning/thinking stage.

How would you read the whole sentence in kana and romaji?

In hiragana:
じしんのたいさくをかぞくといっしょにかんがえます。
In romaji:
jishin no taisaku o kazoku to issho ni kangaemasu.