Season 8: Episode 14 - "Ring Up"

"Ring Up": Ted surprises the group when he reveals he is dating a 20 year old, but he decides to stop seeing her when he realizes they have nothing in common. Barney gets Ted to sleep with her so he can vicariously live through him, but he is horrified to learn that the girl is his half-sister Carly. When Barney reacts by staging a wedding for them, Ted stops him and points out that his insistence on commitment means he no longer has an appetite for one-night-stands. Meanwhile, Robin discovers her engagement ring prevents her from getting free stuff and struggles to adjust. Marshall goes for a tougher image to seduce Lily.