To: Mother in Israel. Marrying your (former) brother-in-law is quite common and 100% Halachicly permissible.
Just this past shabbos, in learning Gemora Yevomas, the Magid Shiur spoke about a case he was involved in. A young man died childless – and the only brother was 4 years old. In order to do Chalitza, you need a godol. Not just bar-mitzvah, but a bona fide godol. The poor widow now has to wait 10 years before she can remarry.