Going halfsies

It just happened. I knew it would happen sooner or later. It was just a matter of when, and how. After months of no activity whatsoever on the bulletin boards, suddenly there is contact information for the other families who used our donor. And suddenly, my twins have 6 half siblings that we know about, all born within 2 months of each other. In my head, the meeting was all very romanticized- We all meet and bring the kids to Disneyworld, and they become like dear cousins to each other. The moms become BFFs and we all live happily ever after. The reality was much different.
There was a flurry of emails in the first few days, as we exchanged names and photos of our kids, and compared their personalities. But I'm left wondering what will happen once the initial curiosity wears off. What kind of relationship will my kids have with their half siblings?

My relationship with my own half-sister is a bit rocky. We lived about a half hour away from each other until I was 10 and she was 3. Then she and her mother packed up and moved halfway across the country. My asshole father did very little to keep her connected to the family, and only flew her out to visit about once a year (now it's about every 4 years). He never went to visit her. I wasn't allowed to make long distance calls, and she was too young to write letters back and forth, so we were not close growing up. As the internet became more widely available, we started communicating through e-mail. But she has a tendency to drop off the face of the earth for months at a time. It's not uncommon to go close to a year without hearing from her. She's a bit of a transient, so I'm never quite sure when or where she'll pop up. One month she's in North Carolina, then months later she'll call from Ohio. When she does re-appear, it's always the same. She wants money. She'll claim its because she doesn't have enough to pay car insurance, or rent, or she needs money to buy a bed because she's been sleeping on a recliner. And I will scramble to send her what little can, because she's my half-sister and I feel compelled to help. Some part of me knows she is irresponsible with money, and that I'm being taken advantage of. If she were just a friend, I would have cut her out of my life long ago. But because we have the same father, I feel a sense of obligation.

I wonder if this is what it will be like for my children. Will they feel a similar sense of obligation to their half siblings? Will they feel like their half siblings owe them anything? Will they desire a close relationship with these other children?
I've been feeling personal pressure on the issue, because I realize that at this young age, it's not really about the kids. It's about the moms. It's going to be OUR ability to communicate and connect with each other that will help shape the foundation of the halfsies relationships to each other. Two of the mothers found each other in a cryobank support group when they were trying to get pregnant. They chat regularly, they've exchanged multiple baby gifts, and they're planning to visit each other soon. I feel like a bit of a third wheel coming into the picture so much later. Suddenly all of my adolescent insecurities have re-emerged. What if the other moms don't like me? What if I blow it for my kids because I'm not as pretty or popular as the other moms?** Will my kids be shut out of a relationship with the half siblings if I don't make a connection with the moms? And in the end, how much of it really matters?
After years of resistance I finally caved and joined facebook, and have become facebook friends with the other moms. Will it ever amount to anything more than that? I suppose only time will tell.

**ETA - I'm not actually worried that they won't like me because I'm "not pretty or popular enough", but I do worry that this is an awkward way to meet people, and I don't want to make a bad first impression.