NM re "His family will in no way include this baby in that support network.... That alone makes me feel sad for baby and puts a divide between him and his brothers."
Well, that is appalling behaviour and I feel in a way that if that is how they are with their grandchildren/nephew's (whatever) brother that your existing kids would pick up on it if you were to keep the baby!
The baby will not notice initially what he does not have. There is a big age gap between your existing kids and the baby and I think in the long run you would have to choose to cut your ex's wider family adrift if they did not accept the baby if you chose to keep him. But they sound pretty horrible to be honest! So no massive sacrifice there!
They do know your ex had numerous affairs don't they? Maybe he got someone pregnant, maybe they had a baby and never told him or maybe they had an abortion. Who knows. Your ex's wider family may well see him as the shining example of a good husband but you know different, I do hope you or he has told them! It is so crap that because you havete physical evidence of an affair in your belly it is somehow the thing that 'trumps' all else. Where as his numerous affairs are hidden!
I am not saying this to influence you either way, just pointing out that at the moment it almost appears as if your ex's wider family's hostile attitude is pushing your decision (in some small way) and t should not! Really when you've given up your baby it will be no comfort at all that your ex's wider family cannot bash you with the fact you had an affair and a baby. Because I think they still will. They will not welcome you back into the wider family. I fear that you are in some way allowing them more influence than they should have.
Your ex's wider family's relationship with your sons is well established enough that they could continue to see family if they chose, IMHO, with your ex, so in many ways I am not sure that the impact would be so great on any of your children. Families with half brothers and sisters are common now, and no one would wonder why baby did not have a relationship with your ex's family.
Re "If I keep baby there are threats of homelessness, reduced time with the kids and a messy divorce that he has the back up financially from his parents for while I don't have any way to finance a defence."
Please do speak to a solicitor. I would consider speaking to the police as well, this is abusive behaviour on his part. You could find out more by speaking to women's aid.
He has no right to make you choose based on him making you homeless (from your own home?), that is utter shite controlling behaviour. There are laws now about controlling behaviour in a marriage, even if there is no physical violence. There is abuse financially because he is threatening you.
I am very sorry but I do wonder how long your relationship with your partner will last once baby is gone. Do you imagine you will be able to pick up how you left off? I am not sure you will be able to do so easily. he will remind you of the baby, and you will remind him.
It sounds like he has a lot to sort out in his own life. And it sounds like his life is impacting on you in a very negative way.
You said of this man and his ex/wife, "They also believe that each child they have should get individual time with them.. It's the way they raised the girls and the way they manage things... All that will no longer be possible if baby is in the picture... I'm not chriticising their parenting one bit it's just that it makes it glaringly obvious that the girls who already have to deal with a divorce also will inevitably miss out on time and individual attention they are used to."
I do really worry that you are literally putting everybody's feelings ahead of your own!
Who cares if his parenting style with the girls will change so you can keep your baby! I doubt it would but even so, really, can you hear this? Two children barely connect to you (maybe you have not even met them) are going to influence whether you keep this baby?
Honestly, either it will change because he will welcome baby and step up to parenting a baby he helped to create, or you/he will leave and the girls will be unaffected (except that they will know their dad could not handle another baby).
Please do not allow yourself or him, or especially the girls, to believe that the choice to keep or not keep the baby in the family (so to speak) was down to the girls or your partner and his ex, and the way they chose to parent the girls.
If the baby leaves your care they may never meet him, I think that would have far more affect on them than the one-to-one their parents choose to give them.
If you genuinely cannot raise him, and genuinely think it is best for him to go to a new family, that is your choice but it should not be about his half sisters and how their dad or mum choose to parent them.
(Sorry if that sounds harsh but really you need to not focus on so many other people - your partner, influenced by his ex, and your ex, are all clouding your thoughts with multiple people who will have no real impact on your life in the future - e.g. wider family who may cut you adrift as soon as your marriage is legally ended, half siblings who you may never see if you and current partner split up.
Please clarify your thinking to what is right for you and baby (ONLY) aside from this (and your care for your own kids) and your current partner (if you stay together) everyone else will be a footnote in your life.