Sorry but I do just need to take issue with the notion that when a child has a sibling they have a 'friend for life'.
I genuinely can't understand why so many people still seem to say this as though it is as true and accurate a fact as saying humans need oxygen.
NO, having a sibling does not give you a friend for life. It MIGHT. It might easily NOT.
I, like those who so blithely say that a sibling is a friend for life only have my own, my relatives and my friends' situations as data. However even a small sample set shows me that of all the people I know, there are some who are still, as adults, best friends with their siblings, many (most) who get on fine with the siblings but have little in common, and some who loathe their siblings and fall out irrevocably or simply avoid each other as much as possible. Of the ones who hate each other now, quite a lot hated each other as kids too, some got on OK ish as kids but fell out later in life over family/parental matters.
Is it so far out of people's experience and - more than this - imagination to believe that many people do not really get on with siblings, either in childhood or later life?
On the original point, I can see many ways that it is harder with more than one, for sure - mostly when they are very young.
However there are small but important ways in which having an only can be trying at times as - depending on the child - you can end up being their constant playmate and companion (even if you have friends over a lot, there are times like eg most school days when it is imposisble to have people there every day for tea etc) and that can impact on the household in other ways. I've adapted to this as much as I can ( our house could be tidier for example!!)
Really though it can come down to personality. I know some very very easy sets of two girls, fairly close in age. I know some very very hard work onlies.
But yes I can see why it's broadly speaking harder with more than one, and like other posters have said I have to say I don't mind people saying this to me, really. I'm quite glad, occaisonally, when I realise that there are many things we have it much easier on! DD is not easy at all and I'm happy to occaisonally be reminded that it could be worse... (!)
But I do wish people would thinkk before spouting mindlessly that a sibling is a friend for life. It strikes me as such a bizarre statement. You only have to glance through the MN threads on AIBU, for example, to realise how very badly sibling relationships can go in later life. Those are extreme, yes, but ime the vast majority of adults I know just rub along OK with their siblings, mostly from a distance - sometimes continents away - and use their partners and/or closest friends as, er, their actual friends for life.