Thank you for your Question.
Allah is extremely loving and kind. If you have miscarried twice there must be an underlying cause. InshaAllah nothing to do with the dua.because your dua was full of love for your brother and his wife.
If it worries you, set aside some good amount of charity and make loads of duas. Speak to Allah and ask him to bless you and your brother with whatever HE knows is best InshaAllah.
Pls also recite dua no.25 from sahifah sajjadiyah. Your brother and his wife should recite it too.