my dd is 14months and around the 50th centile - and I have no probs - she wears bgs and 12-18 jeans from next/asda/tesco and 18-24month vests. I like putting her in dresses though - so then it's just tights - though she must be tall as loads of them end up looking like a vest
never had a prob with ds either, only when he PT and now does a moony on frequent occasions
How about selling on what doesn't work to fund some more bits for her? Or let her wear her sisters clothes...I know that my kids aren't that different in size apart from leg length!