Your question about estate claims depends entirely on the precise wording of your dad's will - as your use of term "second in line" in your question is a bit
ambiguous. If his will stated that everything goes to your stepmother and you were only to inherit something if she pre-deceased him, then being "second in line" doesn't really mean anything if she survived him. If this is the way it was written (and I suspect that is indeed what was said), then unfortunately you do not have any legal claim on any of his assets. Once she inherited them she was free to give them to anyone she wanted to. On the other hand, if your dad had signed a trust document that stated that his assets would go to your stepmother but only for her use during her lifetime, and then pass to you upon her death, you would indeed have a valid claim to the remaining assets.
In other words, unless your dad set up a trust that restricted your stepmother's actions she actually isn't "overturning" anything - she inherited the assets from your dad and once that happened she was the sole owner of the assets.
If this is the case, then most likely there isn't anything you can do legally. Of course, this result probably feels incredibly unfair, and may be very hurtful to you. It also may be contrary to what your father really wanted to have happen - as he may have simply assumed that his assets would be shared with his kids. If he worked with a lawyer in preparing his will or trust it might be helpful to try to talk with that lawyer, though he or she may not be comfortable talking with you. In the end, this result may change the way you feel about your relationship with your father.
The one strategy you might consider is talking to your stepmother's children, so that you can explain to them how upsetting this is to you. They might be willing to consider some kind of compromise sharing of the assets, and hopefully they would understand the feelings behind your reaction. Generally we think it's better if you be clear that this is not a legal challenge, but rather, something coming from the heart. If you don't think they will be open to talking with you, then you might want to recruit another family member or mutual friend to help you open up the discussion. If this is not a viable approach, then perhaps you can talk with someone who was in touch with your father when he wrote up his estate documents, in an effort to better understand why he did what he did.
One final point: if you truly believe that this is not what your father intended to do, you would need to retain a lawyer who could file a claim against the estate, and in that context you could question the lawyer who drafted the will or trust. If your did wrote up the documents on his own, without a lawyer's help, then this option probably won't get you very far either.