I think a mother needs to trust her instinct! While there are things you might be able to do - a background check, going through an agency, a nanny cam - for some people, even that is not enough.
It also sounds like you have family around to help out, and that's a great option.
As they get older, your feelings may change - or they may not. I have several friends who only leave their children with family members. It's just their preference, and I think that's ok.
At some point (school?), we need to leave our children in the care of someone else... but, when they are older - we can communicate with them differently, and you may feel more comfortable leaving them with someone else at that point.