You are being very, very U!
If it wasn't for all of the time and money and LOVE that his step-mam invited in him, your DS would never have even realised that he had a gift and a passion for this martial art.
I can totally understand why you want to be there - you are proud of him and want him to know it - but so are they, and they are the ones who have worked with him to achieve this. It is THEIR moment, not yours!
You can tell him how much you support him, and how proud you are of him, You on't need to be on the spot for this any more than you have been for his previous competitions (there must have been a few or he wouldn't have been invited to take part in this prestigious event). Please, please don't spoil this for them, and for him - they )his step-mum in particular) will know enough about the sport to praise him on particular things and actions, and he will appreciate that - and also, their attention will be solely on him for this day, and not divided between him and your DDs.
Be generous- let them have this wonderful moment that they have both/all worked for. Perhaps there will be a video that they can buy so that you will have a record of his achievement to enjoy in your home and share with friends.
If you insist on going because it's "your" weekend, it will poison your relationship with them forever, and worse (as you may be prepared to do that) will turn his triumph to ashes because of the trouble it causes between you and his dad and step-mam.
And what if, further down the line, you need a swap for a weekend, and you have soiled the nest so much they tell you to stuff it?
Or maybe someone independent could ask your ds, just a simple question if there's 2 tickets and 4 people who want to go. And you all agree to honour whatever he decides
Someone suggested this - while it's fine in theory, in practice it makes him choose between you - don't even think of it. Never make him choose between you unless it's "Would you rather go to the football with dad this weekend or to the zoo with me?" sort of thing.
Just don't. It's cruel to him and will tear him up - and what will you feel if he chooses his dad and step-mam? Whoever is not chosen its going to feel rejected.
Swap the weekend, let them have a great time, and ask him all about it when he gets home.
She sounds a wonderful step-parent - if only they were all so caring.