I am currently learning ~たことにする but I am not so sure how to translate it.
The example sentence is:
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According to jisho.org, the meanings of にする include:
- to decide (on something)
- to place, or raise, person A to a post or status B
- to transform A to B; to make A into B; to exchange A for B
- to make use of A for B; to view A as B; to handle A as if it were B
- to feel A about B
where definitions 2-5 have the form "AをBにする"
We can apply these definitions by splitting your sentence into an "A" and a "B", then thinking about which of the 5 definitions seems the most plausible. You can split the sentence into an "A" and "B" as follows:
Thus, interpreting the sentence literally, the speaker is saying that they want to "transform/make/view/handle" the fact that their family went to Germany while they were in Japan into something that they didn't hear.
My interpretation is that the speaker wants to go to Germany too, so hearing that their family went without them was painful, and therefore the speaker wishes that they had never heard about it.