To tell you the truth.  I'm more than a little underwhelmed considering what the did for some characters and not others.

When I saw that Little Mac had 2 really unique extra costumes I thought for sure everyone would get at least 1. Well there is always DLC! Come on Dry Bowser!

The whole Mario team in there Sports getup.  Putting Pit in Street clothes.  Kirby Hats. Oh Shoot better yet...

http://i.imgur.com/svjqtoT.png
http://i.imgur.com/QV2Ypgy.png
http://i.imgur.com/7RZaGiV.png
http://i.imgur.com/rufe1j5.png
http://i.imgur.com/UYmNHB2.png
http://i.imgur.com/9OQc6Cl.png
http://i.imgur.com/Uo100Mt.png
http://i.imgur.com/Gs3h8hn.png
Heck even
http://i.imgur.com/J3EHPIy.png
http://i.imgur.com/dwLu6hO.png
For an odd alt for someone.

I'm shocked Nintendo approved of letting the Koopalings appear as Bowser Jr.'s alternates. It completely undermines every attempt they've made clarify that they're not supposed to be Bowser's offspring. Miyamoto must be pissed by this.

Miyamoto thought yellow would be a better color for Kirby, Sakurai still kept him pink.  Sakurai has a long history of not caring about what Miyamoto thinks.

I think the story is that they're adopted.

Doing a little digging.  The Koopalings are siblings but, aren't Bowser's kids is the official current line.

Nope. They're simply Bowser's henchmen-- that's always been the official story for the GAME (just as Bowser's Western name has always Bowser in the GAMES). Granted, they were "King Koopa's" children in the cartoon, but the cartoon was always different-- they also all had entirely different names in the cartoon too (probably in part because of the name "Morton Jr.").

Pretty sure they are nieces and nephews to Bowser.  So, they're all cousins.

I don't know where you heard that, but Miyamoto specifically stated himself they are UNRELATED to Bowser. It came straight from the horse's mouth.