An immediate reaction to the revelation about the abuse is guilt. At first you will guilty because you feel as though you failed to protect your sibling. Then after a while you will start to feel guilty that you are trying to deal with your own feelings and emotions about the abuse, you feel this is selfish.
You really have to be honest with yourself at this point. Sometimes you have to admit some responsibility for the abuse. I don’t mean that as a perpetrator, but because you failed in some way as a parent. Perhaps you left your child with a stranger, or you left them alone unsupervised for a period of time. Perhaps you ignored an obvious warning sign, or you didn’t listen properly to a small cry for help and misunderstood the meaning.
If this is the case then yes you have to accept that you were responsible for the well being of your child and as such you have failed in a small way. But, and this is a big BUT, it does not mean you were responsible for the actual abusive act. You had no control over the abusive adult and you had no reasonable idea what was happening.
If you can honestly say you had absolutely no idea the abuse took place and you always made sure your child was supervised properly, even if an adult abused this position of trust whilst you believed they were in charge, you have nothing to feel guilty about.
You can always sit back and go through all of the ‘what if’s and ‘maybe’s, but none of this speculation will help your child now. If you think you may have reasonable grounds for feeling guilty, because of one of the reasons I mentioned above, or you don’t have a reason but you still feel guilty, you must, for the sake of yourself and your child, put this feelings to one side and look to the future. Even if you let your child down then, by missing the abuse, you have a moral obligation to put that right now. You need to be there for them now.
Having said that, it is also important that you choose an appropriate time to talk to your child about your feelings of guilt and perhaps apologize for not being there for them. Between you you will be able to forgive each other and allow each other to continue healing.