First, let’s look at the requirements for Catholic godparents from the Code of Canon Law. Please note the emphasized parts:
§1 To be admitted to undertake the office of sponsor, a person must:
1° be appointed by the candidate for baptism, or by the parents or whoever stands in their place, or failing these, by the parish priest or the minister; to be appointed the person must be suitable for this role and have the intention of fulfilling it;
2° be not less than sixteen years of age, unless a different age has been stipulated by the diocesan bishop, or unless the parish priest or the minister considers that there is a just reason for an exception to be made;
3° be a Catholic who has been confirmed and has received the blessed Eucharist, and who lives a life of faith which befits the role to be undertaken;
4° not labour under a canonical penalty, whether imposed or declared;
5° not be either the father or the mother of the person to be baptised.
§2 A baptised person who belongs to a non-Catholic ecclesial community may be admitted only in company with a Catholic sponsor, and then simply as a witness to the baptism (canon 874, emphasis added).
Perhaps it will alleviate your understandable upset to see that the Church does not consider someone to be “worthy” to be a candidate for godparenthood solely because the person is a Catholic. The person must be a Catholic, true; but he must also live “a life of faith which befits the role to be undertaken.” That requirement rules out prospective candidates who may be Catholic but would also be poor Catholic role models.
You could not be a Catholic godparent because one of the godparent’s roles is to help form a child in the Catholic faith; as a non-Catholic you cannot be asked to form a child in a faith not your own. However, had it been understood at the time of the baptisms, you could have been asked to stand as a Christian witness to the baptisms. Even now, you can and should be a Christian role model for your nephews. If your nephews’ godparents indeed have the problematic lifestyles you report, it would be all the more important for you to be involved in their lives and act as a counterbalance to their godparents’ bad examples through your own commitment to Christian holiness.