"he is getting a partner who will go along with his wants and needs and she is getting a partner who doesn’t abuse/ humiliate/treat her like rubbish, which is what she has had all through her dating years ."
No, she really isn't. He absolutely is treating her like rubbish.
The more I read, the more I wonder if this is not a 'close family member' but yourself that you are describing, because I cannot understand how you can describe him the way you do, but simultaneously not see him for what he is. Look at what you've posted:
"He doesn’t think he treats her badly but the rest of the family does ."
I agree with the rest of the family.
"She on the other hand is absolutely besotted . I don’t have much respect for her if I’m to be honest . She is nice and kind but it’s a pity to watch her all over him and him lapping it up."
Most people (and the rest of your family) will view her in this scenario as nice, and him nasty. 'Lapping it up'. Selfishly taking and not giving (affection, emotion). And YOU don't respect HER? FFS, why do you not disrespect HIM for his appalling behaviour? That poor woman! Hang your head in shame for that comment!
"The other piece of me is happy for him as he has met a woman who puts no pressure on him after years of bad relationships and pressure ."
If he's so sodding asexual, maybe he shouldn't be having any relationships, since they are guaranteed to go bad because he's only in them for his own convenience, not because he cares for the other person?
"They will figure out having a baby I’m sure."

"I worry though that she will leave him after she has her much desired baby , due to frustration, and that would break him."
I worry she won't, and instead descends into depression and ever-diminishing self esteem as it is constantly reinforced that she doesn't matter at all to him.
Your 'close family member' is a total shit. He's taken someone who sounds very vulnerable and is manipulating her. And you sound perfectly happy to witness him in this. Give your head a wobble, and support this poor woman in getting out of his clutches.