K.B.
Maybe the other mom needs some female time and you are all she gets. I understand that because I know I need time away from my daughter but at proper times. Suggest a Moms Night Out without the kids (the dads watch the kids at their own homes not at the same home).
If that is not an option find a babysitter you both can agree on (split the cost, it is cheaper to hire one babysitter to watch 2 kids then hiring 2 babysitters) and go some where while the kids play (not around nap time). This way you know someone is watching the kids yet the other mom gets that time with you she wants.
Or if you really don't want to be around the this other person then don't go over. If you are uncomfortable with what she suggest and does not listen to what you say while she keeps pushing for it then she is not looking for a true friend or playmate for her son. Tell her that you prefer not to leave your daughter unsupervised but would love to sit and talk to her where the children are playing (living room, yard, playroom, whatever).
Also tell her that you need to talk with you hubby(significant other) about when you will be letting your daughter sleep over and do not want her telling your daughter things you have not agreed to. It really sounds like the lady is using your daughter to get what she wants. I was not aloud to sleep over till I was 6 years old and that was same sex sleepover... I really can't remember ever having a sleep over with boys (maybe in 6th grade but the boys were on the same level as the parents while the girls were in the basement with a female adult). I was taught that you do not sleep with boys till you are married (of course this excluded my brothers, I shared a room with them on vacations). Just tell her that you do not approve of sleepovers with boys, especially at this age, and that you would appreciate it if she would stop bring up the topic or telling your daughter things you have no agreed to.
If she gets all huffy puffy about it and does not respect your views boundaries you don't have to be friends with her. You can't get away from being her neighbor so be cordial to her but just tell her this is how I am raising my daughter and would like to be respected for that, you don't have to agree with it but at least respect my rules/boundaries my hubby(significant other) and I have set for our family. Adding that your significant other and you have talked about this will reinforce that you mean business and will not waiver.
Best wishes... I hope you find common ground or part ways without to much fuss.