Hi spanielgirl I have no experience of what you are living so I am only making general comments in relation to adoption.
I am birth mum to a 9 year old girl who is increasing like a teenager (get out of my room, it's up to me if my room is a mess etc) and I have a teenage nephew (who has already gone through the grunting comments phase).
I also have a new son who is 3 and has lost both parents and foster carers and is most certainly grieving that loss (and may do for some time).
I think what social workers will want to see if that even if you do not love your stepson, you have empathy for what he has been through, losing your mum at the age of 4 must be completely devastating and has probably impacted on his life in a very large way. I have at least 4 friends who have lost their mums (because they just left or because they died) aged about 6, 7, 11 and 17 and it impacted them all greatly!
So 4 is just so young to lose such a significant person.
I think it is very understandable if he has put his mum on pedestal - even if she might not have been 'worth it' in others eyes he may well feel she was the centre of his universe, I think at 3 I am pretty much that to my new son so why wouldn't a child put a mum on a pedestal when they lose them. I know rational adults probably do that too!
We all know how difficult teenagers might be very difficult about stuff like doing chores and basically considering anyone else's feelings for some of the time! I have met some truly amazing teenagers so it is not always the case but I remember myself as a moody, spotty 16 year old - obsessed with my appearance in a very negative way, struggling with school work and friendships.
I think what social workers might be interested to know is how your relationship with your stepson has developed over the last 6 years (or longer if you met his dad a long time before you got married).
They might want to know what you would do if you did not love your new adopted child.
One thing people talk about a lot on these boards is the 'fake it 'til you make it' approach where you act in a loving way to your adopted child before you feel that love. I was very fortunate in that the love came very early for me (I am still a newbie, our new son only arrived less than two months ago!). So I have not had to fake it but to be honest with a birth child as well, the love was not instant when she was first born.
There are times when much as I love my kids I do not like how they behave or how they make me feel about myself (this is my dd really because she has found it hard and struggled since ds arrived).
I guess what I am labouring to say is that even if you do not love your stepson have you managed to forge a relationship with him? If the worst he could say is that you nag, and I expect all parents nag, then presumably you do have a relationship and he is not aware of your lack of love.
I also wonder how much you and your husband have tried to cement your relationship as a whole family, after the arrival of your birth child, as well as prior to the arrival of your now toddler? If you have tried and it has not worked, have you insight into why that is?
I think these are the key things. When your stepson leaves home if his room will go to a new child will he feel welcome to visit etc? I think social workers might want to look at that side of things just because if there is tension between the existing members of a family this could impact on your new adopted child as well.
I really hope you do not take offensive, I am not trying to stir things up. I just feel you have a slightly more complex family relationship and you need to know what has worked and what has not, and why.
It may be that you have done everything you could and it has just worked out this way.
You do not mention your husband and how he feels and I am sure social workers will want to explore all of this. I do not think they will be wanting to judge you for your relationships but their concern will be for how this will all work out for a new adopted child, who may well be grieving the loss of birth and foster parents etc (even as a baby not able to verbalise it) and so your understanding will be needed, and all the family to some extent, to help that new person come into your family.
I am basing this on what you have said and there may be all kinds of factors which you have not mentioned - e.g. does he have a very close relationship with grandparents and that is why he might move out or is he feeling a bit pushed out? (Again, please do not take offence I am exploring the issues you may be asked about and I have no prior experience at all).
Best wishes. 