考（かんが）えられない is the negative potential form of the る - Verb 考（かんが）える ‘to consider’. It will regularly be used after the 副助詞（ふくじょし） (adverbial particle)
など (
なんか, or
なんて), or any expression that nominalizes a phrase, like ということは.
考（かんが）えられない is regularly translated as ‘(A) is unimaginable’, or ‘(A) is unthinkable’, which is quite close to the literal meaning.
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20年前（ねんまえ）には携帯（けいたい）で映画（えいが）見（み）たりゲームなど出来（でき）るとは考（かんが）えられなかったでしょう。
It was probably unthinkable that you could watch movies and play video games on your phone 20 years ago.
人（ひと）に手（て）を出（だ）すなんて私（わたし）には考（かんが）えられない。
It is unimaginable that I would raise my hand at a person.
あんな小（ちい）さい人（ひと）が相撲（すもう）をやろうと思（おも）ったということは考（かんが）えられない。
It is unthinkable to imagine a person that small decided they wanted to do sumo.
人（ひと）違（ちが）いだったことは考（かんが）えられないか？
Is it unthinkable that you misremembered the person?
While
考（かんが）えられない refers to the inability to think of a certain thing, 考（かん）えない ‘to not consider’, or 考（かんが）えていない ‘to not be considering’ express something that the speaker has control over. Due to this, only
考（かんが）えられない may be used when implying something that is unbelievable.
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死（し）んだ後（あと）のことなどほとんど考（かんが）えない。
I don't think about what happens after dying.
結婚（けっこん）した後（あと）の事（こと）など考（かんが）えていない。
I haven't thought about what happens after marrying.
妻（つま）と離婚（りこん）することなど考（かんが）えられない。
It is unimaginable that I would get a divorce with my wife.