She'll melt with a sign that says "Love of my Life"

Then when she approaches, get down on your knee with a ring box and have a condom in there instead. Wait until a little crowed forms before you open up the box and say soemthing loudly like "I'm so glad you're home hun, your sister was just not as good as you"

when she runs up to hug you, punch her in the mouth. she will NEVER see it coming. after, many LULZ.

^ I'm gonna do that next time....lol