What is wrong with how I wrote 你也不会呢 in 20年来，没有人相信我。你也不会呢？
20年来，没有人相信我。你也不信吗？
Two changes:

&quot;会&quot; - &quot;信&quot;
&quot;会&quot; is normally used for something that happens later.
Compare these two:

20年来，没有人相信我，你也不会！(No one believes, you won't either)  
20年来，没有人相信我，你也不信！(No one believes, you don't/didn't either) 

Second one is better, as the son said &quot;这不可能&quot; in the previous answer.
And keep the same verb for better coherency.

&quot;呢&quot; - &quot;吗&quot;
&quot;呢&quot; is normally used in a special interrogative sentence (特殊疑问句).
&quot;吗&quot; is used in a closed-ended question (一般疑问句), or a rhetorical question (反问句).
As mom is not waiting for the his response, I suggest having a rhetorical question here.

To check if a rhetorical question is correct, just remove the sentence-final particle (吗)
and replace the question mark with a exclamatory mark (&quot;?&quot; - &quot;!&quot;).

Edited to add a &quot;你也不会&quot; example.
母：“20年来，没有人相信我，你也不会（相信）。”
子：“是什么，告诉我。”
母：“你没有老爸。”
子：“这不可能！”

When she hasn't told her son, it is ok to use &quot;你也不会&quot;, but better to say &quot;你也不会相信&quot;.