For peace of mind - can you contact your friend and see if she is still willing to take your child in the event of your death? If she is, and you are comfortable with it, leave your will as it is.
If she is not, then you will have to cast your net for other guardians. Another close friend, church member, clergy (married pastor), etc.
You could also talk to your sister about your reservations of her being guardian of your child. An open, honest, clear headed discussion for her to answer parenting questions, explain why she wants to raise your son, etc.
I understand your concern. I am a single mother and do not want my son to be raised by his father -who is marginally involved in his life. My will "leaves" him to my Aunt and her married daughter, my cousin. While not ideal, as it has him living with my cousin, and I prefer he live with my Aunt, but she is almost 70 and does not want the day-to-day angst of raising of a teen (heck, who does!), at almost 15, he aware of the arrangements and comfortable with them.
Which leads me to another question - how old is your child? Can you have a playful conversation with your child about who he would want to live with? Excluding Santa and the Easter Bunny, it may be an "Ahah" moment for you.
Ultimately, you have to do what you gives you most peace of mind and what you feel is best for your child. Other people's wishes and feelings really don't figure into it.
God Bless