element_nesting2.html

<div class="parent">
    <div class="generic-child-element">
        …
    </div>
    <div class="unique-child-element">
        <h3 class="title">This Block's Title</h3>
    </div>
    <div class="another-child-element">
        Some text…
    </div>
</div>

element_nesting2.scss

.parent {
    width: 50%;
    float: left;

    &:before, &:after {
        content: "";
    }
    &:first-child {
        color: red;
    }
    &:hover {
        color: blue;

        .generic-child-element {
            display: inline-block;
        }
        .unique-child-element {
            display: block;
        }
    }
    &.open {
        height: 100%;

        .unique-child-element {
            color: red;
        }
        .another-child-element {
            color: green;
        }
    }
}
.parent .generic-child-element {
    display:none;
}
.unique-child-element {
    display: none;
    //[lots of CSS lines…]
}
.another-child-element {
    //[lots of CSS lines…]
}