No; you will be understood, but there are more idiomatic ways to express this thought. Consider instead:
I always found the sight relaxing.
In this sentence, "the sight" refers to the previous sentence, and the sentence structure is simple: I (subject) found (verb) the sight (direct object) relaxing (object complement).
An alternative is:
I always found the sight of them relaxing
Here, there is a more explicit reference to the previous sentence in the predicate, but the object is still "the sight."
If you say:
I always found their sight relaxing
the possessive "their" applied to "sight" suggests that you are talking about the tai chi practicioners' vision, rather than your view of them. In actual usage, most people will understand what you mean. However, it is clearer and more idiomatic to use "the sight" rather than "their sight."
Consider a similar sentence:
I always found her weak eyesight depressing.
In that example, you are clearly talking about an aspect of her. Now:
I always found her sight depressing.
This is ambiguous; the most likely meaning seems to be the opposite of "her weak eyesight" even though the construction is the same. It is less ambiguous and more idiomatic to make this change in meaning explicit:
I always found the sight of her depressing.