I managed to get both sisters knocked up on the downloadable version( don't know if it works on the browser version or not) by editing the MAIDENS_PREG_PATH to 0, That is if you already have one of the sisters knocked up.
1.Save your game file by using the "Save to file" option in the save section.
2.Open the save file in windows Notepad.
3. Click Ctrl + f and type in MAIDENS_PREG_PATH and change the value to zero(0).
After that, you can go back to the shrine maidens and the event to choose which sister you prefer will popup. Make sure you choose the one you haven't impregnated yet.
PS: this trick worked for me but I am not a coder so make sure you have an extra save file laying around in case something goes wrong. Enjoy
1.Save your game file by using the "Save to file" option in the save section.
2.Open the save file in windows Notepad.
3. Click Ctrl + f and type in MAIDENS_PREG_PATH and change the value to zero(0).
After that, you can go back to the shrine maidens and the event to choose which sister you prefer will popup. Make sure you choose the one you haven't impregnated yet.
PS: this trick worked for me but I am not a coder so make sure you have an extra save file laying around in case something goes wrong. Enjoy