My children are aged 29 / 25 / 22 / 19 / 16 - and they are boy / girl / girl / boy / girl.
The older two have always been very close although marriage has changed the amount of time they spend together of course.
The most difficult relationship has always been between my second daughter and second son. They are very different characters and seem to know - even now - how to press each other's buttons to get a reaction.
But in spite of that all five of them love each other fiercely. There are only two left living at home now but they make seeing each other - separately from us - a priority in their lives and they are planning a big holiday next year for the five of them, their spouses and inlaws.
When they are all home during the holidays they do things en masse with their friends and they share friendships e.g. middle daughter's best friend from the age of 12 is coming to my son's wedding. My daughter is working away for a while and her friend checks up on our youngest daughter, invites her out, has met her for study sessions when they both had exams.
There's 6 years between my older daughter and younger son and they have always been inseparable. She used to call herself his "mini mum" and, as a toddler, if he fell over she was the one he would run to. From the time he was born to till he was about nearly every photo of them has him on her hip!
Now they are more or less adults the two younger girls, who are the ones still at home, are very close.