is even more lonely now that their parents are getting old and needing help.
I took care of MIL for years. Met an awful lot of people also careing for elderly/ill/frail parents.
Number one most often repeated complaint -->
ONE sibling does all or most of the heavey lifting.
It is far from uncommon for the ones who don't do their bit to have a nasty tendency to exercise their right of inclusion in the decision making process with rather too much wieght given to what is in their own best interests, rather than taking into account purely what is their parent's best interest. All to often there is too little willingness to bear in mind the reality the carer sibling is being left to deal with when sib. group decisions are being made.
The single sibling left at the sharp end tends to be female.
I have seen so many sib groups that rubbed along just fine until a parent became ill, or frail. And then it all went to ratshit, with frictions and fractures that remain unhealed long after the ill/frail parent's death.
And that's before any will wrangling starts.
That's not to say that having more than one child is wrong, or a less good choice.
More a case that it is impossible to predict what the future holds, and how future adults will act and interact in the longer term when stressors come into play.
Therefore the only good reason for having 1, 2, 3, or 4 etc. children, is becuase you want that number and you have the resources, economic, energy, metal, emotional to give all those children what they need.
What they will/won't be or do for each other as adults requires that most elusive of items ... a working crystal ball.