Well, I'm presuming it's a small company you have, and you won't be paying multi-million pound fees to David Beckham to endorse.
So I'd guess what you need is to treat someone famous, get them to say something genuine and nice about you, then ask them if you'd mind if you quoted them on your website (which can then take on a life of its own).
Very low key... but could have a dramatic effect. And also a realistic approach. If your sister is as good as you say, and she makes a big difference to a sports person, then this is the sort of thing that celebrities will often do without charge.
So... how do you get yourself in that position? Maybe play the long game.
For illustrative purposes (and I'm sure you'll have to adapt this scenario a lot, but you get the idea)... if you want to treat a premier league footballer, then use your current good reputation to approach a League One football team - perhaps via their physio... tell them how good you are, and offer to give three days' free treatment/training, with a view to getting some paid-for work from them if they like what they see.
Once you've got that on your CV, hopefully with a good reference from them... write to every premier league team and offer them the same deal. Maybe you have to take smaller steps (start in League Two... step via the Championship or whatever)
There are plenty of footy teams who engage in 'alternative' stuff - usually out of some desperation to change something when they're in the lower half of the league (which half of all teams are!!). So I'd say you've got a chance.
Like I say, adapt as necessary.... different sport, different approach... but just think through what one of these superstar endorsers would respond to... If they know that you've got a track record with similar people, then maybe one or two might bite.
Good luck