first of all, if this was your father I am so sorry for your loss. Truly, honestly sorry.
secondly, unless you suspect (and have PROOF) he wrote the Will under psychotic or medicated or forced state, then no... you cannot and SHOULD NOT contest this Will.
If this man wrote his Will with good intentions and a good mind, then this is what HE wanted, and it would only hurt him if he knew his bio children were acting greedy and selfish after his passing to prevent other children whom he may have considered "like" his own, or it's possible and likely he loved his step children and like his wife, felt an honorable duty to them. And that is his WISH. So let the man have his wish.
My uncle recently passed away and something similar is happening amongst his children and his 3rd wife (who was an ANGEL caring for him during his last years with alzheimers), and I just know my uncle would be turning in his grave to see his children acting this way over MONEY.