i eat all sorts of things (and i’m on a super limited budget, so it’s all pretty affordable)! i don’t really know what kinds of things your sister likes, but there are plenty of vegan food guides on the internet that might help you out. pinterest is personally one of my favorite places to find recipes (and while not always aimed at the “affordable eating” crowd, they’re generally easy to tweak).
i actually have a pinterest board of veg-friendly recipes (link’s right there)! maybe it could help you? let me know if you have any more questions!