What would the din be if a man were waiting to give his Yavama chalitza or yibum and then instead of doing either he went and married her single sister? Would the zikah have been strong enough that the marriage shouldn't be chal (in effect) at all because you can't marry 2 sisters? In that case he would still have to do yibum or chalitza to his Sister-in-Law. Would she have patered (exempted) his Sister-in-Law from yibum and chalitza and have simply been m'vatail a mitzvat assay. Would he have to give the sister a get & still have to do yibum or chalitza to his Yevama?
The end of the first Mishnah on 41a discusses such a case. It says that the brother must divorce his wife (because of the Isur of Achos Zekukaso), and he must do Chalitzah with the Yevamah (because he cannot do Yibum for she is Asur as Achos Gerushaso). However, this applies only if he had done Erusin with the sister. If he had none Nisu'in with her, then the Zikah with the Yevamah is completely broken and he may remain married to her sister (and he was Mevatel a Mitzvas Aseh d'Oraisa), and there is no need even for Chalitzah (as TOSFOS on 18b, DH Shomeres, states). (See, though, the lengthy RASHBA on 041a.)
Kol tuv,
Y. Shaw