Are the children of my wife from her previous marriage considered strangers to me?
Whoever told you this has no idea of what he is talking about. The children of your second wife hold a special status in relation to you. If you read the Quranic verse 23 of Surah 4 which lists the women a man may not marry, you will find that a man may not marry his wife’s daughter from another marriage once he has consummated the marriage with her mother. In other words, if you have consummated your marriage to your second wife, you are prohibited from ever marrying her daughter; you cannot marry her even if you divorce your second wife or if she dies.
The case of your wife’s son is different because he is not related to you or to your first wife in any way. He is a stranger to your first wife and will remain so.
And God Almighty knows best.