RosieandJim89 in one sense maybe putting DH's biological half siblings on the family tree is not a big issue, and in one sense leaving them off would not be a big issue except it kind of shows that he is isn't able to accept the reality of them because he doesn't know them.
Putting them on the form won't create a relationship where one does not exists. And I agree with others that he should not feel bad or responsible for there not being a relationship.
However, you may have to deal with these 'issues' through adoption, and it is good to know things are resolved and fine.
For example your child, when they come, might have a similar-aged sibling or one who is a lot older or younger than them now - full or half sibling. Or maybe in the future they will have a new birth sibling or half sibling. It's worth thinking about how you might 'deal' with that and how you may help a child to process this. If the new sibling stays with birth family your child may not meet them during childhood, but they will need to know about them. If the child is taken into care they may be adopted and your child would probably be able to meet them.
Just because the child has not seen or will not necessarily see a sibling will not necessarily mean the child will feel the same way as your husband does. I think that may be one area a social worker may want to be sure of. Namely, that your dh will be able to cope with your child having a sibling they may not see etc.
Our son came to us as an only 'child' and we have a birth dd so he has a sister now. But I am always aware that at any time in the next however many years either his birth mum or birth dad may (together or apart) may parent a child. It is unlikely that child would be able to stay with their birth parents if the circumstances are the same as they were when our son was removed into foster care.
So one thing one takes on with adoption is a certain degree of uncertainty. If our son were to have a full or half birth sibling I would do all I could to ensure he was able to have a relationship with the child, if appropriate, in some form. He would definitely need to know about them. I think these may be the kinds of things social workers may be concerned about in relation to this issue. How would these things be handled. I am sure you would handle them well but social workers will want to ask about things for this reason (IMHO).
I think part of adoption is just thinking about these 'issues' and things in our own lives can feed into them. For example if you and your husband adopt and then there is a sibling somewhere, perhaps in the future, will this bring up unhappy memories or concerns for your dh?
Putting people we are related to on our family tree is one way of working through these issues before any child is on the horizon.
It must be possible to get the names of the two 'half-siblings' from someone and if not then you can genuinely tell the social worker you can't find them (but I expect Somerset House would know). You can ask the social worker how to best reference this if your dh chooses to include them.
Hope this makes sense.
All the best.