The truth of the matter is that many of the working breeds (ie Boxer) tend to be same sex aggressive. As a general rule (and this is NOT always the case) dogs who are of a greater age difference seem to have fewer issues but dogs close in age can be a problem.
Honestly - you have 2 options. You can rehome the younger bitch and that is going to be tough - the new home will have to be carefully screened and understand that she must be the only bitch in the home or 2) you MUST crate and rotate. By this I mean, AT NO POINT IN TIME CAN THESE TWO DOGS EVER, EVER, EVER BE IN A ROOM TOGETHER.
I have seen a male dog go through a plate glass window to get to another male and male fights tend to be less violent that bitch fight. You can be sure that this is in no way, shape, or form something to be taken lightly.
IMHO - the aggressor should not have free run of the house, putting the older dog up in a room. At 11 years of age, your older girl is probably not going to be with you forever and would enjoy some time with the family.