How I Met Your Mother Season 4 Episode 11 (12/15/08)
When Ted's sister comes in to visit, Barney does his best to hide it from Barney. However, when Lily accidentally let's it slip, Barney is there waiting at the apartment for her arrival. Ted has no choice but to invite Barney to their dinner.
When Heather reveals she's planning on moving to New York and get a job in finance, Barney offers to help. However, when Heather asks Ted to co-sign her lease, Ted says no. Ted is nervous Heather is going to sleep with Barney and sends Lily to spy, who walks in on them putting on their clothes after their "interview."
After numerous innuendos by both Barney and Heather, Ted calls them out on having sex. They reveal they never had sex and only tricked Lily because they knew she was spying. Ted then realizes his sister isn't just a kid anymore and offers to help her get the apartment. A new member of the gang anyone?
Marshall and Robin, meanwhile, had their own adventure. Robin was homesick for Canada and so Marshall took her to where he goes when he's homesick for Minnesota, a bar where other Minnesotians hang out. She pretends to be from Minnesota so she can hang out at the bar until she beats Marshall's high score at a fishing game. Marshall calls her out for being from Canada and she's evicted from the bar. Marshall eventually helps Robin out by taking her to a Canadian themed bar. Yay!