That's the way they all became the Brady bunch

We've had three year old Ma-chi, the half brother of Pearl and Louis, staying for a few days, with his mother, Anje. So (just to make it completely clear), Ma-chi has the same donor dad as Pearl and Louis.

Anje and Ma-chi live in England, although Anje is German. We'd met Anje on her previous visit to Australia, when I was pregnant and Ma-chi was eighteen months old. From the safety of England, Ma-chi is reportedly quite intrigued with the concept of having a younger brother and sister, although I think he found the realities of sharing toys and attention a little challenging!

We love Anje and Ma-chi. Anje is a lot like us, which is peculiar, because Donor Dan is quite different. His friends are all quite different to us too. They're younger, more closeted, less political, more urban. Anje is a feminist, a university lecturer and an attachment parent, so we connected on a number of levels.

So we all spent a bit of time with Donor Dan and his boyfriend James over the weekend. It's - unusual - creating family as we go along. The gulf is no wider than it is with my extended family of origin, and while we don't have a shared history, our children bind us in a shared future. There are awkward moments. There are bits of each other we just don't get. But we work together. Our relationship isn't about us, it's about the children.

I asked Donor Dan to pick up Louis, who was having some nappy-free time. 'He's just done a wee, so you should be safe,' I said. Poor Danny - I didn't mean to freak him out; I take wee for granted. Poo, now, is still a little harder to deal with.

The boys came over and organised dinner two nights after the kids were abed. They were our most successful nights. The daily outings were less so, partly because I think the boys find the children overwhelming. Immersed in life with two babies, we forget how crazy it can look from the outside. If I put myself in their shoes, I imagine our life looks chaotic, messy, loud and hellish. I imagine they are relieved when they leave us, with our non-stop children and our pumpkin-splattered clothes, to go back to their quiet, orderly, stylish apartment. I certainly watched Anje with the inexhaustible Ma-chi and wondered how I will cope with two three year olds.

We want Ma-chi, Louis and Pearl to see each other as kind of cousins. We've downloaded Skype so they can chat. They can work out their relationship with Donor Dan as they go along. At the moment, Ma-chi is very attached to the idea of Donor Dan. Anje says he often has conversations about his daddy in Australia. Anje is a single mother - I think Ma-chi is trying to figure out his family structure. Our kids will get the standard, 'Some children have a mummy and a daddy, some have two mummys, some have etc etc,' routine, so they may have a different way of making sense of his role.

James has even less guidance about who he is in relation to the kids, but he's keen to be in there. He wanted his mother to meet the children, so we all trooped out there for an overabundant afternoon tea. She had bought gifts for all the children (must send her a thank you note). Our donor's boyfriend's mother feels 'almost like a grandmother', she said, even though she's never met the children before. How's that for extending the ripples of familial connection? We think we're really new and radical and groundbreaking, but really our families are a variation on an ancient theme - the women bringing up the children, the men swanning in occasionally, crones everywhere taking delighted responsibility for the new generation, regardless of blood.