New step-dad, new baby?
So I recently discovered this forum and felt obliged to introduce myself, and perhaps vent a little with a few concerns of mine as a new SD. I've read many of the topics here and many of your troubles and triumphs seem quite similar to my own, although I feel some conflicts expressed here really do dwarf my own.
I'm a new stepdad, officially as of October of last year when my wife and I married. I'm 28 years old and my wife is 32, and I'm the proud stepdad of SD6 from her previous marriage. My wife and I dated for a a little over a year before getting married and before that, I really had absolutely no experience with children. Thankfully, SD6 is very well behaved thanks to my wife who has no trouble putting her foot down on rules that I wholeheartedly agree with. I love them both dearly, and am still quite astounded at how easy it was to adapt to this new situation. SD can be a handful at times, but she is a young girl and aren't they all?
What I have is not really a question but just a concern that some of you may have had more experience with. My wife and I plan to start trying for a child of our own in a couple of months. We've already discussed that we want two of our own children and, since she's 32, we are going to try for back-to-back kids before fertility becomes an issue.
My parents accepted this new SD with open arms and they treat her practically as their own grandchild, which I'm very thankful for. My wife's mother frustrates me at times, however. She married into money and acts as if no one else's wishes matter but hers. Rules go out the window when SD visits her and she gets whatever she wants over there. And then you have my wife's BF's parents, who are pretty much the same way. SD's BF ran off to another state about a year ago and we haven't heard from him since, so SD is showered with attention from at least three different directions.
My concern is that our new child will get nowhere near the same attention that SD does on a regular basis. I mean, look at the holidays. The girl gets a mountain of presents from all three parties and then myself and my wife. I DO NOT want my new son or daughter having anything to do with SD's parents on her BF's side, so there's a real quandary already. My parents have already expressed to me that they will take up the slack for this possible lack of attention, but then I already know that my wife will be jealous of that for her daughter. And even aside from holidays, how do you explain that SD gets to go swimming at her BF's parents house but your child cannot go? How do you adjust for the attention that SD gets from a thousand different directions as opposed to your child who doesn't have as much family?
Sorry if this is long, but thanks in advance for the input and I've very glad to have found this place. 
This is something I dealt
This is something I dealt with, and all I can say is you just do it. Don't worry so much about it, because as long as you raise your children to appreciate things, it will not matter. There will always be things that seem unfair, and all you can do is explain it or balance it out as much as possible. Children need love to feel content. Once the cup is full, they don't benefit from what runneth over, if that makes sense. Good luck with the baby making!
You let them know that having
You let them know that having both mommy and daddy with them every day is more important than anything else...period. It works.
I don't think its going to as
I don't think its going to as big of an issue as you think. There's going to be quite an age difference between your bio kids and SD. In the first three years, your bio kid isn't going to notice that SD goes to her grandparents house and he doesn't. Your bio child will be used to her going to school without him, going to friends houses without him, so going to paternal grandparents house ocassionally isn't going to phase him either. Once he's old enough to see a difference, he will be old enough to understand the family ties. And who know, these grandparents probably won't be nearly as spring and involved with a preteen/teen anyways... She's going to want to be with friends on the weekend, not hanging out at grandmas.
My bio son is almost 3 (birthday is the end of April) and my stepkids are 10 and soon to be 9. They go and do things with the grandparents and my son doesn't think anything of it. He stays with mommy, and the big kids sometimes go other places. Same as the big kids go to a different school, and they can go on different rides at the park, and have different toys..... He just thinks of it as a difference in age, not parentage.
(we have my stepkids full time BTW)
And the grandparents have def cut back on how involved they choose to be as the kids have gotten older. When they were smaller, they'd constantly ask to have them for the day, or an over night. At 9 and 10, not so much. Maybe every couple of months we get asked to bring the kids by. I suspect that by the time bio son is old enough to wonder about it all, it will really be a non issue.
Your baby will still have you two for parents, her parents, and your parents. If paternal grandparents take SD for the day, just do something fin with your kids, if they take SD for Xmas and buy her stuff, get your bio kids a few extra things while she's gone.
The kids will have a 7-8 year age difference, so any inequity will be largely hidden by the fact that they are at such different stages of their lives.