Am I sinful for being unable to conceive, and what is your advice to me after my husband married another and his treatment of me changed, knowing that I still communicate with his family and do not forgive them for their slander?

If the intention is to avoid childbearing, there is no harm in it, and the husband should assist his wife in seeking treatment. Maintaining kinship ties is achieved by visiting the husband's family, and if it's possible to visit them without harm, that is better. Regarding the change in the husband's treatment, he should be confronted, and efforts should be made to remove the causes that are souring the marital relationship. The wife's patience with her husband marrying another is a trial for which she will be rewarded if she accepts and submits to Allah's law. The husband must be just between his wives in lodging and spending, and the wife has the right to demand her share if he falls short.