To give my mom a special gift?

Every year I contemplate what gift to give my mother for Mother's Day. Last year I bought 2 plane tickets to Seattle, Washington, and took my mother. We spent a romantic weekend together; shared a kiss at the top of the Space Needle.. and joined the mile-high club on the return trip. Mom and I have always been effectionate toward each other, but that's usually 2, maybe 3 times a year. My mom has a slim figure and body similiar to the actress Sally Fields... basically a MILF. This year, money is tight and my sister and I are thinking of giving mom a special gift.

She has spoken with our mom, and from mom's reaction to the question: "Would you like to go all the way with us?" ... My sis thinks she and I could give our mom an incest 3-some for a special 2010 Mother's Day gift. I know this isn't common, but if my mom, sister, and I want to, can we view it as normal?