I can empathise as an only child.
First the sleep. DS was a dreadful sleeper, peppered in the early days by chronic asthma after bronchiolitis at 4 months and then repeated ear infections, grommetted at 16 months. It got better when we realised he didn't need much sleep and when we let him stay up until 9.30/10 he slept until about 6/6.30 rather than 8 to 1 and then up until 5.30 - just learning and being raring to go.
He was 3.5 when dd arrived and she needed more sleep and from quite early on I could put her down at .7.30 and she's sleep until 11, then a feed but would then need 2/3 feeds between midnight and 6.30. The one night she slept through aged 11 weeks, I got mastitis!
I don't think I had an unbroken night of sleep until she was settled at school, so for about 7/8 years.
However, the need for another was visceral and like you I was desperate for my DC to have a sibling and had initially wanted three or four children but for us that wasn't to be.
DH is one of three but both his sisters live abroad, one could be on a different planet entirely and there is no common ground whatsoever. Frankly I am glad and if she lived here I cannot imagine my DC wanting to mix with their cousins, cousins I wanted them to have because I don't have any. His other sister, who doesn't have children, is more mainstream. Neither visit the UK very much, the older one doesn't even bother to call her mother.
Ultimately, I have learnt that friends are more important and now dh and I are older, we are effectively both only children and I shall look after my mum and he his. They are both 89 and we haven't had a proper family chirsitmas for three years because neither can be left on their own. 350 miles apart.
All DH got from having sibs to be perfectly honest was less food as everything for 4 was stretched for 5 and arguments over who had more chips and who nicked the Mars Bar from the selection box. His elderly sister told me to think carefully about marrying him because he was a capitalist bastard - it's never in 35 years stopped her from tapping him up.
Our dc are 27 and 30 now. They are closer and always were and in touch a couple of times a week although one has moved abroad for work. DH and I are just back from visiting.