it's not the kind of data you can access unfortunately, as I don't think it's published, it's about who and what you actually know.
As @prh47bridge has said, do check the school admission criteria before you use this idea, as some schools may just say 'a sibling at the school', some may say 'a sibling who would be attending the school at the same time' some may say 'a sibling who is attending the school at the time of the application etc'.
Check you've been put in the correct category, and check your home-school distance is correct (try a few different routes in case you find an unexpected public footpath), as her @prh47bridge instructions.
If the criteria is 'sibling who'll still be at school':
Say you know Bessy in your daughters year has a sister, Jenny, who's in year 13 at the minute, and you know this as her mum has talked at the school gates about how much the girls are growing up, with Bessy moving up to high school and Jenny going off to uni next year. That's something where you could speak to their mum directly and say 'I heard Bessy got into the Girls school, can I ask where you live? My daughter didn't get in, and I'd have thought you lived further away than us' If she lets you know that actually they do live further away, you can check that the school has got your door to door walking distance correct. Go out and measure it yourself, plot it on a map, point out any public footpaths or shortcuts they might have missed. If they've incorrectly applied their admission criteria they have to let you in, and if you can tell them exactly where it's much better for you (eg "Your admissions criteria states that preference is given to girls who have a sibling attending the school who will still be attending at the time of admission, I believe you have mistakenly put a child in this category, Bessy, as her sister is Jenny, currently in year 13, so Jenny will have left before Bessy were to join. This means Bessy should have been in the no siblings, based on distance category, she lives 2.10miles from the school, door to door, my daughter lives a 1.8 mile walking distance from the school, as per this map with the walking route highlighted. Therefore, you have applied the attendance criteria wrongly and my daughter should have been granted a place before Bessy). As mentioned, they can't withdraw Bessys offer of a place, so her mum would loose nothing by sharing this info.