All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
There is no harm in giving the Zakat money for the marriage of the stepdaughter whom you mentioned in the question if she cannot be married off except by that, and if she is not sufficed by the spending of whomever is obliged to spend on her.
If the person who needs to get married is unable to meet his own financial expenses, it is permissible to pay Zakat to him/her and he can be given Zakat that would be sufficient for their marriage without any extravagance as such a person is considered among the poor.
However, one should not be given Zakat if it helps him/her in what is considered extravagance.
The husband is not obliged to spend on his step-daughter; so there is no harm in giving Zakat to her as long as she is eligible to receive Zakat unless he (the step-father) had committed himself to spend on her, either explicitly, or inexplicitly according to the view of some jurists. In this case, he does not give her the Zakat of his money.
Miyaarah, from the Maaliki School of jurisprudence, said:
“Also, the Zakat is not to be given to whomever one had committed himself to pay spend on, as he is not obliged in principle to do so, such as if he commits himself to spend on a step-daughter and the like. Clarification: It means that the one who had committed himself to provide living expenses and clothing to someone becomes obligated to spend on that person just like the person who is in principle obliged to spend (on those under his guardianship). This is applicable whether his commitment to spend is explicit or the actual situation is that he is spending on them, and this is whether or not it is a relative; this is stated by Ibn Abd As-Salaam.” [End of quote]
If one is permitted to pay the Zakat money to marry off his stepdaughter, then there is no specific percentage. She is given from the Zakat money whatever fulfils the need in marrying her off without any extravagance.
Allah knows best.