I don't trust my sister-in-law to watch my kid.
What is your reason for feeling you can not trust her.
I just feel she is irresponsible. I also do not like the things she chooses to do with my son.
If you feel that this person is not trustworthy enough to watch your kid then you are probably making a good choice. If someone leaves any sort of doubt in your mind to watch your child then you have the freedom to not allow them time with your kid. It is completely alright to be weary about who your children spend time around.
I feel guilty about it, though.
If you feel uncertain about her watching your kid than you have every right to act accordingly. Your child is your responsibility. It makes sense to only allow trustworthy people to take care of them.