Can I add my sisters to power of attorney?

Usually when there's more than one sibling as POA it's because a sibling gets their nose out of joint over not being named as POA. But having more than one POA can get very confusing when the time comes to actually utilize POA. Having one designated sibling as POA is much less complicated and if there are sour grapes over POA then the other sibling(s) will just have to get over it.

Do you mean your mother's POA? Is this for health or financial? The short answer is No, only she can designate a second or third person. That doesn't mean your sister's can't help though, there are lots of things you can get done without a POA.