We’ve approached evening visitors by saying a cut off time, and try to give a time slot for others so they don’t stay too long. For us we say ‘ we’d love to see you but we will be busy with tea until 5.30, and then start bedtime at 7pm so we would only be free before that.’ Could you say something similar and suggest that seeing them on the weekend would be better and offer a window of a few hours when you would be free?
honestly, there’s nothing wrong with how you’ve just explained the whole thing to us - id just straight up speak to her and explain the exact same to us, eg you used to meet up late before you had baby, but you’ve got baby now and they come first. you’re trying to create a routine and as part of this you don’t want visitors after 5pm. there’s nothing rude/impolite about it and you’re just being totally honest.
You could say: “Hi Sis (whatever you call her) I’m trying to get _____into a routine and I’ve noticed he sleeps best between ___and__pm. I know we’d planned to have you round at that time but are you able to pop round any earlier before his bedtime starts at ___pm. Could we move his aunty/nephew cuddle time to maybe like 5 ish after work?