It’s not inseparable, but as Manda Jo says, you don’t really have many memories of being a child under five and it’s entirely possible your mother did what I did and waited to have actual conversations when someone else was around or you were asleep. I surely didn’t think my two kids under five would both be quiet and refrain from dangerous activities while I was on the phone. It’s one thing to expect a 6 or 8 year old to entertain herself while mom is on the phone for a few minutes . It’s another thing to believe that three children under five will do it for the same few minutes. Because remember, three under five probably involves one being under three.
But there’s something else that occured to me- I’m not sure how old you are, but your siblings are quite a bit older than you. Is it posssible that there’s a bit of a “generation gap” between you and your fiancee and his siblings? Not the generation you belong to , but rather, the ones each set of parents belong to. When my son was young there was a fairly wide range of parental ages among his classmates. I was just about in the middle, there were a couple of mothers 10 or 12 years older than me, and one or two ten years younger. Even ten years made a difference in parental philosophy and twenty made a much bigger one. To give a made up example, it was as if the older parents insisted the child eat what was served or go without dinner, the group in the middle expected the child to have a fix a bowl of cereal or a PB&J if they didn’t like what was for dinner and the younger parents just didn’t serve foods the kids didn’t like. You might want to discuss general parenting philosophy with your fiancee without bringing up his siblings.