Okay if you sister didn’t know herself (i.e. if my wife didn’t know herself), that’s a different case and I wouldn’t be as upset then, but again, I guess that’s just me and my nature: If you need to do ANYTHING, just be honest and say so. I will respect your wishes. I guess I just don’t like to be kept in the dark. And yes, I will be very protective about my daughter, but I am still not sure if I will do official checks. I may act like your mom (i.e keep my eyes open and everywhere) because that’s what all parents do. I am a straightforward person. If I need something from someone, I tell them straightaway. Oh well, that time is too far, don’t make me think about her marriage, let me enjoy her infant days 
I know anything is possible…and I guess you haven’t been in a situation where family members lie just to take revenge. Without getting into too many details, here’s soemthing:
My mamoo was engaged to his father’s cousin’s daughter. Mamoo’s FIL was running in the local elections and mamoo’s dad was an influential person and could have a very postive impact on his votes. Engagement was done but after the old man won the elections, he broke off the engagement. Then my mamoo’s family started looking for another girl and found one in the next town. The girl’s family wanted to do checks (without telling) and came in their neighborhood to ask questions about mamoo. Unfortunately, they ran into this family that broke off the engagement. You can imagine what kind of feedback they gave and the other family backed off. Then there was another rishta ad before it was done, this same cousin family secretly went to their house and badmouthed mamoo and his family and that rishta also didn’t happen. Then finally he got married to someone else.
Point being: This is the kind of experience I am speaking from. If your cousin can do this to do, imagine what other relatives can do to screw things up for whatever reason.