You need to be able to spank both sisters, for me I spanked Alice 3-5 times and Lisa 2-4 times to get the convo with Ann about private spanking, first time she will refuse so wait a day and go and ask again.
Okay, sometimes you can not get the option to spank Lisa.
If you only get the option to "watch" or "take it yourself" then you must punish Alice first, it took me 5 times of punishing Alice before I got the option to spank Lisa to open up the new content.
This is with the "You Got To Fight For Him" Lisa GF path, (its the one I am doing at this time)