DokiDoki sister Help

[kikakiki75]

Recently I try to take the True ending for the sister route, but during the final Doki Doki the sisters start to switch very fast, only after a shot, so it's impossible to me to raise the bar to 100%. Any solution?

[Azzmosis]

Several Years too late but:

You can hold I believe, R1 down to charge the shot, doing that with every shot from the start seems to make the sisters swap about a lot less, and charges the bar a lot more than a normal shot.
Also as you scroll through picking an area to target, the Japanese characters that appear are different sizes, aiming for areas where the characters are bigger will charge the bar more.
After shooting an area, choose the next area on the other side of the body to avoid being blocked.

Should be fine maxing out the bar with those tips.