There is nothing wrong with you having breastfed your sister's daughter and your sister having breastfed your son with the aim of making them mahrams, because this is a permissible aim and there may be a need for that. It is not haraam, as your husband said.
There are two conditions for breastfeeding that establishes the mahram relationship:
1. It should be five or more breastfeedings, because of the hadeeth of Aa'ishah (may Allah be pleased with her) who said: Among the things that were revealed of the Qur'aan was that ten definite breastfeedings make a person a mahram, then that was abrogated and replaced with five definite breastfeedings, Narrated by Muslim, no. 1452.
2. That should be within the first two years of the child's life, because of what is proven from the Prophet (blessings and peace of Allah be upon him) in the hadeeth of Abd-Allah ibn al-Zubayr, that the Messenger of Allah (blessings and peace of Allah be upon him) said: There is no breastfeeding except that which fills the stomach. Narrated by Ibn Maajah (1946); see also Saheeh al-Jaami, no. 7495. al-Bukhaari (may Allah have mercy on him) said in his Saheeh: Chapter: One who says: There is no breastfeeding after two years because Allah says (interpretation of the meaning): The mothers shall give suck to their children for two whole years, (that is) for those (parents) who desire to complete the term of suckling' [al-Baqarah 2:233].
Breastfeeding means that the infant takes the nipple and suckles from it, then lets it go by himself, in order to breathe or move and the like.
Based on that, the five breastfeedings may take place in one sitting.
If your sister's daughter breastfed from you five times, then she has become your daughter through breastfeeding and a sister to all your children, and to any other child who breastfed from you five times.
Similarly, if your son breastfed five times from his maternal aunt, she has become a mother to him through breastfeeding and he has become a brother to all of her children and any other child who breastfed from her.
The mahram relationship is not limited to the girl and boy asked about; rather the girl becomes a brother to all of your children, present and future, and she is a daughter to your husband and a sister to all of his children from any other wife, if applicable.
And Allah knows best.
Reproduced from Islam QA