Yibum of a ben Tesha - the Ramban (Yevamos 51B) says that it makes her a full Eshes Ish for everything except Misah, even to become Asur through Znus.
I am almost certain that there is no Drasha about a Katan making Kinuy in this entire Maseches. On 27A, there is a Drasha Ish-Ish to include Cheresh, Shoteh and Shuamum that Beis Din warns his wife - why is Katan omitted?
By the Sugyos of Gilgul Shvua (Kedushin 27...)- why doesn't the Gemara say that Kenusah is a wife of a Ben Tesha (and this is the source of Gilgul)?! (There should be Gilgul, since she becomes Asur through Znus).
This seems to me a proof that Yibum of a Ben Tesha is d'Rabanan, or (as the Ra'avad) mid'Oraisa, but she cannot become Asur through Znus.
The Pashtus of the Mishnah in Yevamos 96b and the Gemara there is that it is mid'Rabanan. See Tosfos Kidushin 19a DH u'mid'Oraisa.