Given the Ottoman tradition of becoming sultan and having ones brothers assassinated: I wonder how they insured the lineage if the sultan was unable to sire heirs, as unlike in the west, there where no uncles to claim the throne.
The murder of brothers was allowed, yes; but not every prince committed the act. Sometimes, when an heir claimed throne, there was already a brother or uncle in the picture no one really cared about. Mustafa I, Murad IV and Ibrahim the Deranged are the first ones that come to mind.
Another thing is, Ottomans weren't really a big fan of marriage when it comes to the head of the state. Meaning that the sultans produced offspring with concubines instead of getting a queen. This practice assured a strong and healthy supply of heirs.
In case a prince happened to be sterile, that probably wasn't a big problem; as most of the time princes did have children before they actually ascended to the throne, and the ones that did have heirs would secure more support(especially from their father) to dominate or get rid of the opposition.