If a woman did not wait three months after [her divorce or widowhood from] her husband, but remarried and gave birth, and it is now known whether it was a nine-months' child by the former [husband] or a seven-months' child by the latter —- [if] she [also] had [other] sons from the first and second [husbands], these [sons] perform chalitzah but not yibum. Similarly, he performs chalitzah for them, but not yibum. [If] he had brothers by the first and second [husbands] not through his mother, he may perform either chalitzah or yibum, whereas they —- one [must] perform chalitzah [after which] the other one [may] perform yibum.