Praise be to Allah.
1. We praise Allaah for having guided you to Islam and we ask Him to complete His favour by helping you to be steadfast and by guiding your family to Islam.
You are doing the right thing by not attending the religious service in the church, because taking part in religious proceedings of non-Muslims is, at the very least, a major sin, and could lead to kufr.
With regard to attending your brother’s wedding party where alcohol will be served with the food, we advise you not to take part as you say. Attending such parties is haraam and is a major sin. Allaah says (interpretation of the meaning): “… then after the remembrance sit not in the company of those people who are the zaalimoon (polytheists and wrong-doers, etc.)” [al-An’aam 6:68].
The Prophet (peace and blessings of Allaah be upon him) said: “Whoever among you sees an evil action, let him change it with his hand [by taking action]; if he cannot, then with his tongue [by speaking out]; and if he cannot, then with his heart [by feeling that it is wrong] – and that is the weakest of faith.” (Narrated by Muslim, 49).
When you take part in such events, you will not be able to change the evil actions with your hand or your tongue – in most cases – so all you can do is change it with your heart, and you cannot be really doing so if you are taking part.
The Prophet (peace and blessings of Allaah be upon him) said: “Whoever believes in Allaah and the Last Day, let him not sit at a table where wine is being drunk.”
(Narrated by Ahmad, 14241, and classed as saheeh by al-Albaani in Irwaa’ al-Ghaleel, 7/6).
The reason why it is forbidden – as it appears – is that sitting with them implies that you condone the evil action.
4. Apart from the fact that these things are forbidden, if the brother makes these compromises, this will weaken his efforts to call his family to Islam. The more sincere the daa’iyah is towards himself and to His Lord, the stronger his efforts to call others will be. But if he compromises and vacillates, this will make him look less trustworthy in the eyes of others.
And Allaah knows best.