It could be that your partner's discovery may seem very exciting and in some way a fresh start for him in terms of his parenthood. As time passes, the novelty will pass away and his relationship with his 'new' daughter become much more like that with her younger sister, with all the ups and downs of being a father.

You don't say how old your children were when you got together, but I assume that they were not very small, and it's worth remembering just how hard it is to bond with children who may well remember their real dad, as well as possibly resenting your attachment to him. I this case I know of what I speak.

What a shock for everybody, and no wonder you are upset and feeling unfair about it. The situation hasn't been fair in that there have been secrets and still are a bit from what you say. I am a great believer in openness and honesty in families, most particularly between the parents or step-parents. Step parenting is a tough job at the best of times and it might be a good idea to see a family therapist for some guidance with this.
It is natural for your partner to dream of happy families, but in general children are not delighted to find there is an older sister or brother, particularly if their feelings about it are not carefully attended to.
In the end the adults have to come to joint decisions and need to agree the way forward. I hope you feel less depressed as the time goes on.