@Surprisedbuthappy
Talk about an attitude straight out of the 1950s! It's obviously the hussy woman's fault for having children with different fathers, isn't it? I think it's far more like the absent father abandoned his family than the OP just decided to move on and start a family with another man for the hell of it.
This is rubbish. I never said anything about a hussy woman - that's your projection. Fact is best predictor of future behaviour is past behaviour and if a woman has left father of Child 1, there's at least a risk that she'd leave the father of Child 2. Even you must accept that - there's a risk and a greater risk than someone who has never been married at all with no children or someone who has been married for life.
I made expressly clear that in some circumstances there may be exceptional reasons why it isn't a risk - a woman widowed or where there is unforeseen trauma/mental health breakdown that lead to unexpected change of character (war veterans for example)
Typically two people who choose to have a child together but fail to stay together to give that child a stable family background are each at fault - even if the fault on one side is to fail to take long enough to assess the true character of the other person as being compatible with them and family life. It's very rarely all the fault of the "bastard" man any more than the fault of the women.
The point I was making is that its not that unreasonable to
firstly not consider a "step niece" part of your family as unconnected and unrelated
and
secondly, that no one really longs for someone they care about to find a long term relationship with someone who comes with the complication of existing children from another relationship - and all the kind of crap (that this thread exemplifies) that goes with forming new relationships when they are existing children to consider. Your hope would always be they'd find someone single to create a family unit together for the first time and stay together for life, happy and stable.
thirdly it's not unreasonable to be very sceptical about how long the new relationship will last such as to put you off forming a quick and close bond with the "step niece" because of the risk of the relationship with the sibling failing too.
That's all. No hussy women. Just probability that there is a risk of the new relationship failing too so that rushing into bond with the step child is an area to enter into very very slowly even if you have an inclination to do so - not just for your interests (if the wife buggers off you may never see the step child again) but also for the child (unhelpful to get fast bonds if the family splits).
& given that the OP is threatening to divorce the brother over this issue, it might not be that off the scale here.