Sigh
At the moment, the OP's DD has mum and dad at home.
Mum is throwing a fit because her DD's baby sibling was not ignored and rejected by her dad as the mum expected to.
Now, she may end up breaking up with the dad because she is jealous of this child of his.
One of the arguments was how do you explain to a 6 year old that dad has another child?
My argument is that if addressed properly children can easily cope with it.
That it will be better for her to deal only with the news of the sibling rather than with news of a new sibling and the news that her parents are splitting up (it doesn't matter if at the same time or not), or even finding out when she's older that her mum and dad could treat her sibling so appallingly as to ignore this child and lie to her about it.
In an ideal world bad or potentially upsetting things don't happen, but most adults and children can cope with them.
Finding out about a new sibling, even outside of their parents' mariage is not that bad.
So, my point is that OP's DD is not really the issue here.
It's mostly how she will find out that is the issue. Will it be dealt with by her parents together and welcomed into the family unit, that sadly is not "perfect" anymore, or will she have to deal with more than this issue too?
However, if OP's selfishness about this means that she'll treat her OH badly about this child, and should the child visit her home (as it would normally at some point) if she treats the child badly, then it's best if she leaves her OH.
Just hope for your child's sake that the roles are never reversed... and your now OH doesn't ignore your DD in favour of a new woman.