Let me wrap my head around this...
You are jealous because you aren't pregnant and you want your child to have the spotlight, so you don't want to conceive until that attention is guaranteed. But you want to have the baby now because you are feeling broody. However, because another baby would be a second pregnancy, you feel as though that child would not get the same amount of love and attention as your sisters baby - who is currently pregnant with her first child.
D'you know what? This kind of reminds me of the wedding dilemma. One person in a group of friends announces a wedding just before somebody else announces theirs, and both weddings are around the same date, requiring the same bridesmaids.
It also reminds me of when I announced my pregnancy to a few close friends - one of them was also jealous because they weren't pregnant for whatever reason.
I know that it's not exactly the same, but it sounds to me like what you are experiencing is competition.
A lot of women feel in competition with one another at some point, it's a natural thing. It's human.
So, no. IMO, you aren't a horrible person. It's okay to feel the way you do, we are entitled to feel how we want. It's how we act on our feelings which defines us.
What you need to do instead, is try to understand why you feel the way you do and what you can do to help combat it and help yourself through it, whilst at the same time reassuring your sister - because if you fake it, she will surely pick up on it eventually lol, she's your bloody sister!
Sisters are often competitive of each other, and as the younger sister growing up, you will have watched your older sister do everything first. You will have watched your parents give that all-important feedback for the first time to your older sister and you will have learned from that, rather than the direct attention.
This leads me to assume that because you had a baby first, it feels like you finally did something your sister hasn't done - you had a baby first. You experienced that feedback from your older family members first. Whether or not you consciously registered it, it was probably quite nice to have those emotions, which weren't second-hand for once. It might have felt like something was exclusively yours for a change, because it was a new experience for your parents.
Have you thought about having a chat with your sister, or your OH about how you feel?