I am so sad to hear you describe yourself as homely and overweight. This sounds like you put yourself down for your very being and are uncomfortable with who you are. There are no homely people, just different from us looking people or different looking from the crowd we are used to being around. If you judge yourself harshly do you also judge others this way? Perhaps read into things were nothing offensive exist. You did say only full siblings and their partners are invited. This means half siblings, such as yourself are not invited.
To think being homely and overweight as one reason not to get invited to a wedding is an assumption that actually puts you down as well as your step sister because you are accusing her of being superficial. We've been to glamorous weddings (pain in the bum) where everyone dresses up and I have discovered "everyone cleans up" nicely for those no matter their weight, how they perceive themselves or how casual they might be on a daily level because of the more formal attire, make up etc.
If you truly were the only step relative not being invited, which it does not sound like since you said they are inviting full siblings and spouses, something else is at play and you may want to ask your step sister if you have done something to offend her. If you know you have not, then just assume no step siblings are going, it's the norm and no offense was met. In our family dynamics the step siblings met later in life as teens and young adults, have little in common and are holiday friends. No biggie. And it also sounds like if you deem them glamorous to your dowdiness, the comparisons can't help but feel awful and other people pick up on such awkwardness.
If you have decided your step sister is a superficial bitch who is snubbing you but you don't want to ask her directly if you have offended her then you will bring that attitude to every gathering she is at and you will bring unwarranted drama to the family dynamics. Don't go there. Assume no step siblings are invited, be glad to not spend the money to go, enjoy your precious life and lose the weight so you can feel awesome about that aspect of your life- been there (excessively hard work), done that and it truly is a worthwhile effort.
There is a great book called Self Compassion by Kristen Neff and I highly recommend it to people who put themselves down and suffer from worthlessness. It's easier to blame others for how bad we feel about ourselves, including making a step sibling out to be the problem because of this or that, then it is to do the hard work of making ourselves the person we wish to be. Most of the difficulty lies in not recognizing our role in keeping ourselves down through excuses and lack of compassion for self and others. Instead we let anger run our lives. Don't go there. Your step sibling has every right to invite whom she wants to her wedding which means being angry or hurt is not an appropriate response if you believe in her autonomy and even your own when you make a decision that potentially hurts another's feelings, though unintentional. I recommend you send her a beautiful engagement card, if you haven't already, wishing them the best time at their upcoming wedding. If you've sent an engagement card then just send a lovely wedding card congratulating them and wishing them the loveliest of days. This helps keep the family dynamics flowing in a way that doesn't create harmful drama, helps you mature in your compassion and responses to being hurt and helps everyone know you as a mature, confident woman- great example to your children and your family.