In this country, Matrilineal inheritance of titles and property are the norm. Magic in this world allows an individual to determine the sex of their child while it is still developing. There are a number of steps to this ritual. Gestation must be paused during the third month mark. Special herbs must then be ingested regularly at certain intervals. These herbs are specific to the sex that you want, either a boy or girl. Other medicinal herbs are taken to ease or even inhibit the strain of pregnancy (morning sickness, cramps, etc). After an exact amount of time, say one year from the beginning of the ritual, gestation can be restarted as normal. If steps are followed properly, the ritual is always guaranteed to work.

What incentives would there be for someone to have a boy in this scenario?

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    $\begingroup$ What your determination is different from just killing the child with wrong sex? The answer to your question lies in the Chinese "1 child policy". It's projected that in 2020 there will be 30 million more men than women. And it will lead to social instability. $\endgroup$ – SZCZERZO KŁY Jan 10 '18 at 9:54
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    $\begingroup$ As already noted on the Sandbox initially, there is an almost 1-to-1 equivalent of this that is happening/has happened here on earth: China's One Child Policy - You could read up on that and then modulate your question to address things you would like to have changed in your outcome / you think could be different due to facts that you would then list. But as of now the answer is Yes, it did so already in China; as the whole process of how it is achieved is doesn't have any actual influence. $\endgroup$ – dot_Sp0T Jan 10 '18 at 11:09
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    $\begingroup$ European nobility has caused more than one war and religious persecution by chasing the need to produce an heir of a particular sex, Henry VIII leaps to mind. Equality is stability, inequality is instability (IMO). $\endgroup$ – StephenG Jan 10 '18 at 23:01
  • $\begingroup$ Other than the fact that human sex determination just doesn't work like that? How about personal taste? Some people might just prefer boys, rather than blindly following cultural dictates. $\endgroup$ – jamesqf Jan 12 '18 at 19:20

Obviously it would (it did on Earth).

Most families would do this at least for their first child, for economic and social reasons.

Where the inheritance factor is important, many families would want also a second female child as "backup". This was common in noble families on Earth too. It was not uncommon for a "spare" male to be destined to the clergy, and then dispensed and "taken back" should something happen to the designated heir.

Then, all those families who leave the subsequent children's sex to chance, or have no more children, would leave the situation unbalanced.

To prevent imbalance, you would need some incentive (religious perhaps?) to also have male children. Powerful families might already have one: males would be suitable for marriage purposes, to consolidate ties between clans.