The Poskim say that even in a situation where we would allow Yibbum for example the brother has amputated legs above the knee Rachmana Litzlan and how much more so when we don't allow Yibbum and he did it anyway, after the brother has done Yibbum he must divorce her straight away.
נקטעה רגלו מעל הברך, לדברי הכל חליצתה פסולה; ועל היבם לייבם את האשה ולהוציאה אחר כך בגט בשו"ת שרידי אש, ח"ג סי' מט, בדעת גדולי האחרונים - שבות יעקב, בית מאיר, חת"ס, הגר' חיים עוזר גרודזנסקי, וכן כתב באו"ז סי' תרסה, בשם ריב"ן
After Chalitza if that brother or any other brother want to marry her through Kesef Shtar Biah (see Kidushin 2a) with Eidim (Yibbum no longer applies Yevamos 10b "Shuv Lo Yibane"), the Marriage does take effect as this relationship is no longer Chayav Kareis (see Kiddushin 66b) But it is a forbidden relationship which is either Chayav Asei (transgressing a positive Commandment) according to Shulchan aruch Even haEzer 162,2 and Rashi Yevamos 10b, or Chayav Lo Taase according to Tosfos and Bach Beis Shmuel and Beer Heitev. There is a 3rd opinion of the Rambam that there is only a prohibition derabannan to marry one's Chalutza see Beis Shmuel here