I adopted my DD2's little brother.
Originally I was asked in 2005 whether I would consider adopting him and said no, and then after 13 months of delay I was asked again 'just to make sure, since your circumstances have changed' in 2006, and after a good month of soul searching and agonising and every emotion under the sun, I said yes. and brought my DS home in early 2007
You have all my sympathies and support, it's a very hard thing to be asked. Lovely for those people who don't have to think before saying 'yes' but it doesn't work that way for most of us IME
I would say that first and foremost you have to think about the family you have now, and seriously consider whether you could cope with it. 5 months is a very small age gap indeed, and you need to think about your own desires and capabilties.
How hard are you finding it at the moment with a new baby and your other children? Remember back to when you brought your DS home, and what that was like, and you would be repeating that but this time with a tiny age gap between new DC and second youngest DC.
How do the rest of the family feel about a new child? I wouldn't have adopted DS if my DD2 was not completely on board, and DD1 felt very positive about it as well. Also how are your kids coping with the new baby so far?
Ultimately any new child changes the dynamic in the family. Do you like how it is now? Were you planning any more birth DC's? If you weren't and you and DH intended your baby BC to be your last child, then you need to think very carefully, because surely you had very good reasons for deciding that
If you do have to say no, then don't feel guilty. You know, I really doubt your DS would be upset with you in later years if you had to say no now. He might feel upset about not being raised with birth siblings (although he might not) but I think it pretty unlikely he would direct that upset at you. Any reasonable person would understand if you said no, given your circumstances.
And yes, as you said, it's possible his birth mother will have another baby. You can't take this baby, and the next and the next. Mind you, my DS was the last child.
For me, I don't regret adopting my son, not at all, he's an utter joy and i love him with all of my heart. BUT, it was really really really hard at first. He was grieving and struggling, DD2 was struggling and showing it with pretty challenging behaviour. I had moments when I thought 'I'm not sure I made the right decision' and then shouldered on. Nowadays it's so much easier and they love each other so much
but - being raised with DD2 has made some parts of DS's life hard. She is not easy to live with. So if any of your children have special needs or challenging behaviour, it's worth thinking about the impact that would have on a new child.
And also, DS says clearly that he does not love DD2 any more than DD1. It doesn't matter to him that DD2 is his birth sister and DD1 isn't. He thinks about them in the same way, he loves them the same. He actually said he doesn't care that DD2 is his birth sister (he's just not interested in birth family full stop)
DD2 says she thinks it's great that she has been able to grow up with her little brother and they weren't seperated, BUT she doesn't love him any more than she loves her sister