If you adopt a little sister then kill another big daddy, will the little sister that you adopted leave you / get mad at you? What if you already have adopted a little sister and dont want to leave her for another one, do you have to harvest the other little sister or can you just leave her alone ?
No she won't get mad at you and leave you. However, there are no practical benefit in keeping a Little Sister once she's done harvesting the 2 additional bodies. It's your choice to either harvest her or free her (both sides having their own perks and cons). If you come accross another Little Sister, you can, if you want just leave her here or you can finish with your current one and adopt the new one. Be aware though, that there are bonuses if you harvest/save all little sister in each area.