You are feeling this for a reason. Sorry about losing your pops at such a young age. Very sad.

BUT does he treat SD more like the wife? Do you feel left out? 

We need more info to be able to help you. Is your SD a mini-wife? Has your DH put her in the wife role and relegated you to something inferior? Do a search for "mini wife" on here and see some stories to determine if it's a mini-wife issue or not. I'm a veteran SM who was a distant second to a mini-wife for decades. Whenever I broached the subject, I got gaslighted and had my past as a very absued child of a malignant narcissist father thrown up in my face. Don't fall for that. But first, give us some details about the moments that trigger your jealousy. 

It's good you are asking yourself questions about why you feel like this. Self examination and exploration can be helpful. I find writing my feelings and thoughts out, then coming back to read them later helps me get a more objective view of whether I'm being balanced. If you think you might have lingering issues from loosing your dad consider looking for a good counselor. Finding a solid friend to talk to can help as well. Strong emotions are easier to understand and deal with when you have positive support. 
Knowledge is also your friend. Reading up on maintaining emotional health can be good 

It might also help to ask yourself "do I have key needs that are not being met? Do I still feel loved and secure in my relationship? If not why? What's missing?" It might be less about the time DH spends with SD and more about he's not spending as much time with you as in the past. It's not exactly the same thing. He doesn't have to neglect one to care for the other. But maybe after being together for so long you don't do as much together, express affection or appreciation like in the beginning. Jealousy often comes because we feel a lack or void, especially if others seem to have what we're longing for. What do you need to feel complete and whole?

Also, having a feeling doesn't make you a bad person. Don't beat yourself up.
 

Hugs!

It may be that you are seeing, in their relationship, something that you didn't get to have (past age of 12), and that brings up a lot of feelings. So, maybe you are not jealous of SD, per se, but it brings to the surface what you missed out on, and what you will never have, again.

Being jealous of an SD does not always means she's a mini wife, or that your husband is not meeting your needs, or that something is missing in your relationship, it's not always somebody elses fault. So, I agree with looking inward.

I could tell you stories about my dad's wife, good and bad. Not all step mothers are saints who have been wronged.

Im reading this book right now, and this very topic came up. She points out that sometimes the SM becomes jealous of the Skid, due to the fact that they did not get time or a healthy relationship with THEIR father, and watching the skid get something they never got and never will get illicits these feelings.

Dont feel bad - this does happen and it doesnt mean you are a bad person for having these feelings.

But some great questions were raised - so perhaps its some kind of behavior via SD and your partner - more info is needed to determin this.

What I am reading about is the competitive nature of the step family dynamic. Often times the kids feel like they are "losing" their parent to the step. And the parent feels guilty (because divorce) and will overcompensate because of this, causing an unbalance to exist.

Hard to say with what you posted.

I 1000% can relate to this!!! When I met DH SD was only 9 and I had very few issues with her or DH (Then my SO). Yes some things here and there but definitely no feelings of jealousy. Now that she is almost 17 I can't stand when she is here! DH doesn't treat her like a miniwife but she does things that to me a teenage girl doesn't do with her dad and are what DH and I do when shes not here. 

Perfect example tonight: After dinner when I was finishing cleaning up DH went to sit on the front porch. Well he and I do this a lot after dinner and drink some wine. Well SD ran right down and ran outside to sit with him. It just bugs the S*** our of me!!! Of course I have no desire to go out there now. I mean he and I can't talk about things we normally can when she;s not here. I have desire to hear her boring stories and all I can think is "Why isnt she out with friends on a Friday night?" 

I know he has to spend time with her when she is here but why cant they watch tv instead of looking like a couple on the front porch??? (FYI this is also what the couple across the street does. Just the two of them in the morning or after diiner) I swear she ran out there knowing I would have sat with him. 

I have 31 more weekends with her here till she turns 18 and trust me I count every single one down.

(Oh and I had a very good relationship with my dad until he passed away at 90 when I was in my late 40's)

It's odd because of the age, the dynamic is just different with divorced dads and sd's overall...but at that age I would be anywhere BUT with my parents chilling out for sure.  Now, as an adult, I love the memories of sitting with my dad as he aged before he passed from cancer.

I was a very rebellious kid, I wasn't very close to my parents at all from ages 14-21.  I was very independent.  I always wanted to be with friends.  THAT said, we did have some very memorable trips with my parents during that time that I treasure so much.  

Keep in mind, we lived in S. America, and twice (16 and 18) I came up to the US and stayed with family here for about 6 months.  So at those ages I went 6 full months w/out seeing my parents or brothers. 

That said, I know other parents who ARE close to their kids during those ages, and though it seems more common in divorced homes that the attachment gets weird, in nuclear families it's very common to see closeness.  My brother and wife are a nuclear home, their parenting was exemplary and their kids are amazing and successful.  He was always very close to his daughters, his son and him butted heads more when he was a younger teen.  They vacation a lot, family dinners at home, hiking trips, game nights....my niece graduated college and moved to her first apartment and is 23....but she comes home to her parents several times a week for dinner or to hang out with them.  It's a tight knit family for sure.  Then again my niece had friends or boyfriends and had her own life, she travelled overseas for a few months during a summer.   

As an example, either of my nieces going outside to hang out with their dad in the backyard would be a very common thing if that makes sense...but they also have a strong sense of identity apart.  I think that's where your situation is weird, as SD doesn't really have that, so all they have is the weird overly attached style and no balance from it.  I still think you need to exit stage left pronto