How to select siblings of another parent when input is focus?

Question

Without js only css?

Answer 1

It isn't clear where this other parent is in relation to the focused input.

If this other parent was a sibling to the selected input, you could use the ~ or + combinator. For example,
input:focus + p {
color: red;
font-weight: bold;
}

That would select the paragraph immediately after the focused input so something like this:
<input>
<p>This will go red when the above input is in focus.</p>

~ is for general siblings instead of immediate next siblings.

If this doesn't involve siblings, I think you'll need to use JavaScript. I don't know of a way to combine the existence of some element in the document with a completely independent CSS selector.