How do I handle this maturely?

I joined a gym recently. My main trainer is very handsome, built, attentive, loves his work, etc. Therein lies the problem, though of course no fault of his own. We have a relationship in the sense that he's the nephew of my late ex boyfriend.
Last night while there a lady worker/student told me that I wouldn't be able to handle his class because it's rough, and he'll push me out if I'm not up to par. I mean, in another class of his he did do that because he felt I needed to build my strength/endurance. Yes, I was disappointed but since he's the boss I went back to strength training.
I really feel the reason for her effort of getting me out is because he's very attractive, built of course and with such a people oriented personality. In other words, as they say he's "hot." So of course I might be in the way.
Yet, it's not known how I am seeing I just started. I am VERY strong willed/minded, meaning basically I do what I want, save for purposely hurting people's feelings. So I went to his class anyway, I saw her, she did a double take, called him over, said something to him, he walked away laughing. Then he came over to me smiling, and began helping me.
I don't want any trouble there. I have no intention of leaving, so feel that if they want him, well do what you have to do, but Im going nowhere.