Hey there, chef!
WELCOME to our forum, it is great to meet you!
Im very sorry to hear your sis is having such a hard time with feeding her baby. So many allergies!

How very nice of you to want to help her out and make a dessert for her. Im sure she will be thrilled!
Like Molly already said, coconut and rice/oat milk are great soy milk subs. I definitely recommend trying them out. Oat milk is my fav and very easy to make at home, too. Just blend up oats and water and strain through unused panty hose or cheesecloth.
Here are some desserts that should fit your criteria that I have tried and loved. I hope youll find something for your sis to enjoy!

Please let us know how things turned out for you and what you ended up making, Id love to hear about it.
Vegan Coffee Ice Cream (use other flavoring or decaf coffee)
Banana Bread - Lightened up - Spelt & Vegan
Vegan Vanilla Cake (use the oat/rice milk option)
Vegan Fruit-Butter Bars
Banana Chocolate Chip Bread Pudding (Vegan) (use the oat/rice milk option)
Simple Vegan Pancakes (maybe top with coconut whipped cream and fresh fruit)
Whipped Coconut Cream (I have not tried this, but many people claim it is great!)
Peanut Butter, Banana, Chocolate Chip Brownies (Vegan or Not) (substitute sunflower seed butter for pb and use rice/oat milk)
Chocolate Truffles (Vegan) (sub sunflower seed butter for nut butter or use mashed chickpeas stiffened with a bit of powdered sugar)
Chocolate Chip Vegan Cookies (For Non-Vegans Too!) (use a non-soy based margarine)
Loula