Have you read the manga and watched the anime as well...you have to do both...I get the feeling that Kohane really loved Watanuki and it's the same for Shizuka - ( shizuka is doumeki,i just call him by his name Shizuka, because there's too many DOumeki )...You know when Watanuki took over Yuuko's shop, he has to pay the price for that which is to never leave the shop and he will never age - ( and i think he can live for a very long time as well, just like Yuuko did ) and because of that Kohane and Shizuka knew that they can never be by his side forever ( and these two really cared for him )that's why they decided to marry and have children ( even though they don't actually have feelings for each other...but I'm sure they care for each other just not in that way )..when you read chapter 212 - 213 of the manga you will see shizuka and watanuki talking to each other, but that's not shizuka anymore that's Kohane and Shizuka's great grandson who looks exactly just like him...just like how Shizuka looked like his grandfather ( Haruka )...I kind of think that Shizuka is being reincarnated again and again to be with Watanuki's side...I think you can tell that, that's not Shizuka anymore...you know how Watanuki doesn't get mad at him...and he actually does some cooking for Watanuki...which Shizuka never actually do...and Watanuki said that.."You really do look just like him, your great grandfather" - ( he means Shizuka )...Then the great grandson said "I resemble him that much"? then Watanuki said something which I didnt understand then after that Watanuki said "Well he looked just like his granfather himself. It might be that kind of lineage". SO that means that Kohane and Shizuka died a long time ago and left Watanuki with their great grandson so he won't be alone...They really cared for Watanuki, and I really think it's a bad ending...Watanuki is still waiting for Yuuko for more than 100 years now...and he's been inside that house for a really long time...if Shizuka and Kohane hadn't marry and had children...Watanuki will be alone for a long time...well he still got Maru and Moro and Mokona.
The link to the manga
link
link
Hope this helps
