Is it permissible to financially assist a lazy brother to establish a new project, without guaranteeing its success, or do destitute families have a greater right to this assistance?

There is no harm in helping your brother with another project, even if you do not expect it to succeed. However, wisdom dictates studying the project to protect money from loss. The existence of poor families does not prevent you from helping your brother; combining both matters is better, especially if these families are relatives. For charity given to a poor relative is both charity and strengthening of kinship ties.