Raepeerk, it's not a glitch. The kids age according to when they were born. Like my oldest daughter (in real life) was 2 years and 8 months when my second daughter was born. At first you could say she was 2 years older, but then when she had her birthday she was 3 years older. It's the same with the game, sometimes the kids appear to be 3 years older than a sibling and sometimes 2.
Also, about the kids looking alike, we have had that happen in most of the games going on at my house. I had a girl, Sonya, then had twin girls that looked exactly like Sonya. My daughter Jez has a family with 1 girl and 5 boys, and four of the boys look exactly alike. In fact, out of 5 games going on right now only one of us has no look alike kids. I wouldn't mind the look alikes in my game if they were only cute. Sonya was the ugliest little girl, then I had twins exactly the same. Urgh!