I had a similar conflict of feeling when I was pregnant with my second baby. My first was a girl, and my second a boy. People kept asking when my baby shower was, and I would look at them puzzled and say "It's my second baby..." and then I would get laughed at, as though I didn't know that second babies get showers too! One of my friends put it quite simply "You are having a baby shower because EVERY baby needs to be celebrated!" So I took it.
If it helps think of it this way, you didn't ask for a baby shower, you didn't organize said shower, and it's given by people who want to celebrate a new birth! Not to mention you said your twins are boys and you are expecting a girl now.
My stuff was that I had all girl stuff, all purple and pink clothes/dresses doesn't go so well with a little boy, and now most cribs are converters, my daughters crib was being used as a toddler bed, so I couldn't hand that down either. I did register with my second, but I also send out notes saying that all we really needed was boy's clothing, and that the register was mostly to give an idea of colors that we liked. It was a nice small shower and we got a lot of cute things!