its funny, I've contemplated this with my own DD, but from the POV that as older parents, a sibling happening naturally just wasn't on the cards & at this age even if it was, theres no way I could cope with the baby stage again :(
this leaves us in a better place to give her more attention & as a result she's a very bright well rounded child, we also can afford more exotic holidays with 1 DC, so she's well travelled & we've been told she would qualify for a Scholarship to private school, which we are considering.
but at some level, as 1 of 2, both me & DH would prefer it if she was too :( but we also happily accept that wasn't our lot & we are very lucky to have her, many aren't so lucky
but I do at times worry for her future & know she would LOVE a sibling, but I have come to the conclusion as I'm estranged from my own Brother, who TBH has been nothing but cause trouble for as long as I can remember & still does that siblings aren't always what they are cracked up to be
something she does say herself after visiting her friends with "annoying Brothers or sisters" lol
So making an actual decision - private ed or sibling - NO!!!! I wouldn't do it, I don't actually think any of us really can, decide to try maybe, but thats about it, nature will make her own decision for you 