We were playing at the greenbelt next to our house with all the cousins and took Bailey's training wheels off to see if she could do it. Sure enough, she could. 
And not to be out done by his sister, Jack wanted to try too. Actually, he just really liked his cousin Porter's motorcycle bike. He did pretty darn good for his first time (especially since he was just 3 years and 2 months old).
Our cousin Easton learned too at the same time. It was a group effort, with everyone picking up fallen riders and getting them started again. We had lots of fun with the whole group over at the park as my little house didn't fit 8 adults and 13 kids very well.
One of the down falls of growing up in a family so close in age with your siblings... you don't even get your own post when you learn how to ride a bike! Bailey thinks it's so cool that they did it together. It is one of the things we wanted though, when we decided to have our kids close, to have them all doing the same things at the same time. Really, we've taught 3 kids to ride a bike in 11 months. Now if we can just get all 4 potty trained...