How can I apply a style to a parent of a specific child

I've created a menu where if the user clicks on one of the side menu items then the side menu and the side menu children are all displayed, but the problem I have is that all my menu items are bold instead of the selected child and it's parent.

I hope I explained this clearly,

My HTML

<div id="second-menu" class="collapse navbar-collapse menu_two">
    <ul class="nav navbar-nav inside-nav">
        <li class="sub_menu">   
            <a href="#">Section 1</a>
            <ul class="sidenav_wrapper">
                <li class="sidenav_item sidenavlast">
                    <a href="#">Section 2</a>
                </li>
            </ul>
        </li>
        <li class="sub_menu">
            <a href="#">Section 3</a>
            <ul class="sidenav_wrapper">
                <li class="sidenav_item">
                    <a href="#">Section 4</a>
                </li>
                <li class="sidenav_active">
                    <a href="#">Section 5</a>
                </li>
                <li class="sidenav_item">   
                    <a href="#">Section 6</a>
                </li>
                <li class="sidenav_item sidenavlast">
                    <a href="#">Section 7</a>
                </li>
            </ul>
        </li>
        <li class="sub_menu">
            <a href="#">Section 8</a>
            <ul class="sidenav_wrapper">
                <li class="sidenav_item">
                    <a href="#">Section 9</a>
                </li>
                <li class="sidenav_item">
                    <a href="#">Section 10</a>
                </li>
                <li class="sidenav_item sidenavlast">
                    <a href="#">Section 11</a>
                </li>
            </ul>
        </li>
    </ul>
</div>

My JS

    $('ul.navbar-nav li').hover(

    function () {
        $(this).find('.sidenav_wrapper:first').css('display', 'block');
    },

    function () {
        $(this).find('.sidenav_wrapper:first').css('display', 'none');
    });


    $(".sidenav_active").parent().css({
        "display": "block"
    });


    if ($('.sidenav_active').length > 0) {
        $('.sidenav_wrapper').parent().find('a').css({
            "font-weight": "700"
        });
    }

My CSS

.menu_two {
    border-left: 16px solid #dfdfdf;
    padding-left: 0;
}
.sidenav_wrapper {
    background: none repeat scroll 0 0 #e7ecf5;
    border-left: 6px solid #8fb8e6;
    display: none;
    padding: 0;
}

.active_sub_menu a{
    font-weight: 700;
}

.active_sub_menu .sidenav_wrapper{
        display: block;
    }

.sidenav_active a{
        font-weight: 700;
    }

My jsfiddle: JSFIDDLE