M.O.
I'm not going to say whether you should try or not, but this is a good time to pause and think through your reasons for wanting to find her.
What kind of reunion are you expecting? Consult with a few people close to you. Do they think your expectations are realistic?
What kind of problems are your dad and stepmom anticipating? Think way down the road in terms of what those problems will mean for them. Does your desire to find your sister still trump that?
I say all this in part because after he remarried and had my half-sisters, my dad pretty much deliberately drifted away from my (whole) brother and me. This was hard for us for a while, but ultimately it made me recognize what a fantastic family I had with my mom, my brother, and my extended family on my mom's side. This has helped me really cherish them and has made me a better daughter/sister/granddaughter/niece/cousin/aunt than I would have been otherwise. So I'm really grateful for the whole thing. But at the same time, I don't particularly want my half-sisters to get in touch with me, and I'm hoping my son and his cousins won't ever seek out their mysterious long-lost grandpa. That would just lead to a lot of pain for my wonderful mom.
So, think through the consequences for others. Think through whether your expectations of a reunion are realistic. I'm not saying don't do it, but proceed with caution.