I've just thought of another example (sorry i tend to think in examples in cases I haven't experienced)
MIL- she is one of eight
except she is only one of two
Her dad slept around a lot (in the end her Mum did a bunk as it happened and left them all).
MIL knows who some of her siblings are but refuses to have anything to do with them because they are- I use the phrase to quote her, not the OP- Illegitimate . She also has no contact with her sister as her sister had an affair, or her son (my DH) as when FIl left her he refused to side.
I am not sure if there is any moral or anything, but I do think had she been raised with a full family she would be less superior (I'm the legitimate one, my way is RIGHT) and would be much happier and less inclined to alienate people. From what i can see, she has gradually alientated all around her.
I'm not saying you're alienating poeple BTW- just that she does.
Life is a complex thing and can throw some strange balls at us. If something awful happened and your child lost you / Dh wouldn't you feel better knowing he or she had a sibling out there he could share genes with? I don't mean live with or anything, just someone who feels a part of them. I dunno, you see adopted people (thinking of FIL here) and so often they are desperate for that bit of identity. I do think it comes after you sooner or later, and if you have a hand in building a good relationship between siblings now at least you'll get an input.
In some ways I think you're strong- I couldn't accept it if Dh behaved like this, and I do love him very much. I also happen to think ypu'd be better without him b ut of course I don't know either of you in RL and can't really judge beyond the narrow perspective that we see on here. I hope you're getting counselling for yourself not just as a couple- Heaven knows I would need it reagrdless of whether I stayed with DH or not!
When you read the things on here, the posts that make you ouch the most, ask yourself why. I dont mean the argumentative ones that MN does so well: the other more down to earth ones. This child is a part of your life now, and as a result so is this other women. Mending your relationship does mean an acceptance of that I am afraid, painful as that must really be. The name etc things might hurt but they're all just symptomatic of so many other hurts and either they're resolved properly now- with acceptance being the only route I suspect- or you will have a lot of pain ahead of you.