As long as the discipline does not interfere with Christian beliefs, it should not be a sin for a Christian to practice qigong. Although some religious leaders are cautious, qigong is not a religion and can therefore co-exist with an actual religion without sinning.
Yes, a Christian can do Tai Chi. There is no prohibition against Tai Chi in the Bible or any other Christian religious text. The advice for Christians from most churches, is to be aware of potential conflicts in philosophy and belief. But Tai Chi as a physical and meditative practice is fine for Christians and Catholics.