Should i tell her?

so my friends and I met in marching band last year. For the past few months I've been talking to her step brother and he is the most sweetest person I've ever met and I have feelings for him. The problem is her step brother told me not to say anything to her cause he doesn't know how she would react and probably try to keep us apart. But idk if she would really do that and I've been wanting to tell her.

Her step brother is older than me by 2½ years older than me and we have a thing. But we didnt do anything sexual things together and I think thats what he is afraid of if his step sister finds out.