All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
First of all, we thank and praise Allaah Who guided you to Islam and we ask Him to make you firm and steadfast on His Religion until the Day you meet Him.
We advise you to seek beneficial knowledge, perform good deeds and befriend righteous and pious women, as these are the best things that help a Muslim woman remain steadfast on her religion.
As regards your question, you are not sinful if you do not love everyone in the same manner. Moreover, you did well by treating your sister-in-law in a nice manner and supplicating for her guidance. Forgiveness has a great status in Islam and this is clarified in Fatwa 84351.
Your sister-in-law should be advised, and it should be clarified to her that it is forbidden to desert a Muslim fellow without a sound reason as clarified in Fatwa 84976.
With regard to what you read about the Prophet that he did not speak to Hind bint ‘Utbah at all after she embraced Islam because of what she did before Islam, then we have not come across any evidence about this. On the contrary, it is reported that the Prophet spoke to her at different occasions. For example, it is reported that Hind bint 'Utbah said: "O Prophet of Allaah! Abu Sufyaan is a miser, and he does not give me and my children what is sufficient, except what I take from him without his knowledge." So the Prophet told her: “Take what will suffice you and your children within reasonable grounds.”
Allaah Knows best.