For some families, they're everything, whilst for others they're irrelevant. So do cousins really matter?
Jude Harris, 32, grew up with over 20 cousins, but she'd only see them at big family occasions and she's still never met some of them. "I'm not in contact with any of them now. I don't even know which ones are married or have children," she says. "As far as my own kids are concerned, I still think cousins are insignificant. It's just a label for what is usually a pretty distant relative."
But Michelle Barker, 27, disagrees. She has just two cousins, but they're as close as siblings. "My mum and her sister were as thick as thieves, so my brother and I would see her two kids - our cousins - loads. Sundays, holidays and fun days out were always with them and I loved it.
"Even now, not a week goes by when we don't get together and they're such a positive force in my life that I can't imagine life without them. But my brother probably won't have children and I find myself feeling really jealous when I see Facebook pictures of someone's kids with all their cousins because my kids will miss out on that special relationship."
Psychologist Dave Spellman says cousins are like any family relationship, in that they can add a sense of belonging that can be really valuable – but, on the other hand, we can't assume that just because someone is family that they'll automatically bring joy to your life.
"As kids, we tend to take the cue from our parents," he says. "So you'll often find that the closer your parents were to their siblings, the greater the chance you had of being close to your cousins. If, on the other hand, there's some big family conflict, it stands to reason that you probably didn't see their children much or ever and even if you did, you probably associated their family with negative thoughts."
Geography is relevant too, of course. If your cousins live in Scotland, whilst you live in Cornwall, there's not much opportunity to form meaningful bonds. "With families being far more dispersed around different areas of the country than in the past, it's much less likely than at any other time in history that people will see their cousins regularly," says Spellman.
Other variables that influence the closeness of cousins include the age gap and personalities, he says.
It's not inevitable that a big age gap or dramatically different character types will prevent the cousin relationship from blossoming, though, as Tammy Philips, 40, knows. "My cousins were five and seven years older than me and whilst I was introverted, they were gregarious and adventurous.
"As a result, there were no two people I looked up to more in the world and nothing excited me more than a trip to see them. In turn, I think they liked looking after me and they found my shyness cute. We became very close and still are today."
In many families, cousins never get the chance to form loving relationships because they're constantly being compared, says Spellman. This can be where rivalry between aunts and uncles gets played out through their children or where parents simply want their own kids to look up to their cousins.
So if you find yourself saying things like, "Your little cousin Freddy is the lead in the Christmas play this year and he's head of his football team," you might find they can't actually stand the sight of Freddy.
Conversely, if you find yourself saying things like, 'Thank goodness you've got manners, not like your cousins who can't even be bothered to thank us for their Christmas presents,' you might find you get a similar result.
These are obvious examples, of course, but Spellman points out that children pick up on the nuances of family dynamics too. "My mum never actually said anything bad about her brother and his wife and children because she wasn't that kind of person.
"But you just knew from her tight lips and raised eyebrows that she didn't care for them and that made me feel like I was being disloyal by playing with their daughter, who I actually rather liked," says Helen Macaskill, 35. "I have since learned that my cousin felt exactly the same."
By chance, says Helen, she and her cousin wound up at the same university. "It was there that we really got to know each other and we've been best friends ever since. I'd definitely say the cousin part of our relationship is relevant to that friendship because it's nice to share grandparents, have shared childhood memories and sympathise with each other about our mad family.
I think cousins are in a good position to do that because they're one step removed from siblings. I saw it written once that nobody understands your crazy family like cousins do, and I do think there's some truth in that.
I was even older when I became close to my cousin. At 23 years old, she's 19 years younger than me and although I'd seen her a few times while she was growing up, it wasn't much. But three years ago, when she had her first of three daughters, I went to see her and we hit it off straight away. Since then, we've become so close that she's moved to our village and we see each other several times a week.
Other people only get to know, or reform relationships with, their cousins even later in life – sometimes in their old age, says psychologist Jean Smith. "You tend to become more reflective about your past, the older you get, and you often have more time on your hands too. So you'll often find older people investing more in family relationships that perhaps didn't mean much to them ever before or they've let go during their busy adult years."
But what about families whose kids don't have cousins? Do they miss out? Not at all, says Spellman. Growing numbers of people think of their close friends as family nowadays and it's inevitable that some of those friends will have kids of similar ages.
I know me and my best friend's kids have taken to referring to themselves as cousins and, given that she and are like sisters, it stands to reason. But if your child does really have cousins in the family, maybe it's time to make the most of it.
Are you and your children close to cousins?