I'm not kidding! I figured it out! And I'm blogging about it for two reasons. First, when the mother is finally revealed, I can go, "IN YOUR FACE!" because I will totally be right. And secondly, so you can marvel at my awesomeness for figuring it out.
So, just as a warning, spoilers ahoy!
I am a huge fan of HIMYM. I was excited to watch the pilot when it first aired, I have seen every episode since, own all the DVD sets, and it is undoubtedly my favorite show (and you know how I love my TV).
In the last moments of the pilot, it is revealed, "and that's how I met your Aunt Robin." In that instant, my first theory as to who the mother is was born: it's Robin's sister. I adamantly believed that the title of "Aunt" for Robin wasn't just a title. I thought that she must actually be somehow related to the kids and that meant the mother must be somehow related to Robin. I thought the title of aunt and uncle for Lily, Marshall, and Barney was a decoy. Marshall only has brothers so no mother there. I think it was mentioned once that Lily had a sister, but I'd have to imagine that Ted had met her already. And Barney just has a gay, married, black brother so that didn't add up either. It had to be Robin.
In Season 2, we met Robin's sister and my theory went out the window. I stopped focusing on my related theory and just sat back and enjoyed the show.
Over the next couple of seasons, we didn't learn that much that tied the mother to the group. We learned who she wasn't, that she had a yellow umbrella, and that she was at the same St. Patrick's Day party as Ted.
In Season 5, she becomes a little more concrete. We know she's a college student because she was in Ted's accidental first class when he got the building number wrong.
Season 6 is when things really start to develop. Between the season premiere and the season finale, we learn that Ted meets her at Barney's wedding. This means she has ties to Barney or his yet to be identified bride.
With that amount of information, she could be virtually anyone. The key to the mystery actually comes on the DVD set for Season 6. The DVD set features a bonus on Disc 3 called, "What we know about the mother". At first, it's fairly straightforward. We know it's not Victoria, Robin, or Stella. We know she's Cindy's roommate. Then, they show the joke from the Season 6 finale when Ted accidentally gives an orchid to a girl in a coffee shop, says she's the mother, then calls "Psych!".
And you expect it to end. But it doesn't! It cuts to a girl whose face is blocked by a yellow umbrella and Narrator Ted explains how the mother wouldn't have liked the orchid. Her favorite flowers are lilacs and Ted was lucky enough to give her lilacs on the day they met.
Why would Ted give her lilacs on the day they met, you ask? Well, we know Ted meets her at Barney's wedding, where Ted is the best man. The only reason you would give someone flowers at a wedding is if she was in the wedding party holding a bouquet or she was close family and got a corsage. And if you look at scenes from Ted calming Barney down before his wedding, you can see lilac in their boutonnieres! Lilacs must be one of the wedding flowers!
Rise! Rise my theory that she actually is related somehow! If she were related, it would make sense for her to be a bridesmaid or just to get a corsage (at my wedding, the close relatives got corsages). The only problem with this theory is how is she related? My original guess was that the bride was Robin and we find out that she has a half sister, fathered by her cheating father. We've already learned that Robin's dad had some mistresses in his past so it could fit.
But this theory didn't sit well with me. It just didn't fit well with the tone of the show. However, a miracle happened in the middle of the night and I solved it.
The mother is Barney's half-sister! Jerry's daughter!
It all adds up! We know she's away at college when Barney meets the rest of the family, Columbia perhaps? (I tried to find a clip of her description, but failed.) It would make sense for her to be at the wedding. It would make sense as to why Ted doesn't meet her until then because Barney only found out about her a couple of months ago.
I still believe that Robin is the bride, too. It would tie everything together well. It would be a great reason why the series started with Ted meeting Robin. If Ted hadn't met Robin, Barney wouldn't have met her, they wouldn't have gotten married, and Ted wouldn't have met the mother at their wedding.
So try to disprove me. I dare you! But trust me, when the series does finally end, if it ends how I'm calling it, you will never hear the end of it from me.