You and your family sound like utter douchebags. If she hasn't already declined the rishta after your mother said she'd be conducting checks on her/ her family, you should decline for her sake. Shallow af.
They obviously had an issue in their family which they weren't very proud of obviously, maybe still emotionally hung up about it and there you are like "oh I fancy her, but her sister ran away, should I decline?! Sob." You don't sound like a Singh. And if you are, you need to fix the f**k up. Do you not have a sister at home? Imagine if some prick turns up to your house to check out your sister and then his thick as s*it mother tells you she's gunna do some checks on your sister. I would slap her silly. You ain't ready to get married mate. You don't even know how to respect someones daughter.
WT F is it with these men in our kaum these days?! Do you all not have sisters and mothers?!?!