Dinsdale, if I might offer answers to your questions which grievously PO’d your sister.
First, I take no offense in your asking these questions, as long as you’re honestly interested in finding out, rather than just needling someone with different beliefs out of some smug and smarmy need to put down people of faith. You seem to be of the former, rather than the latter, so here goes:
Jesus provided the answer (at least in part) to the question of what the Holy Spirit is for. He told his disciples before He ascended that He would be sending a Comforter, i.e. the Holy Spirit.
The aspects of the members of the Trinity are tough (if not impossible) to get a handle on, but since Jesus (God in the flesh) could not remain on earth, there needed to be a permanent and more pervasive presence of God for His followers. This is the Holy Spirit, who always existed, and is no less creator than God the Father and God the Son (“Now the earth was formless and empty, darkness was over the surface of the deep, and the Spirit of God was hovering over the waters.” Genesis 1:2). It would take a better theologian/metaphysicist than I (since I’m neither!) to explain in any more detail. But I hope this helps on that subject.
For your second question, on why God needs to be worshipped at all, the quick answer is, He doesn’t. He desires our worship, but He doesn’t need it. Worshipping God is for man’s benefit, not God’s. Corporate worship is important for a body of believers, to keep them focussed on God, on doctrine, on their missions within the church and in the world, and to uplift and edify each other.
To use a really weak analogy, I could be away from home for 11 months out of the year, never see my wife or kids except for 28 days and I would still be married and be the father of my children. But it wouldn’t be very good, would it, for all concerned parties. Spending a lot of money on churches and statues and paintings is, in one point of view, misguided. But it is also an outward demonstration of the love, devotion and worship given to God. In that respect, it falls far short of what He really deserves from us.
That said, you are spot on regarding the lack of works in the Lord’s name that many church-goers are guilty of. But one does not supplant the other. A balance must be struck between the two. One of the most inspiring things I’ve ever seen in a church is a large banner over the inside of the front door. Everyone who leaves the church sees this message: “You are now entering the mission field.” Words every Christian should live by.
Hope this enlightens you, Dinsdale. I kind of wrote it on the fly, and I know I only scratched the surface. But your questions were good ones, and entirely legitimate for anyone to ask, including (perhaps especially) followers of Jesus.