"[we] love each other" could indicate sexual involvement or be a perfectly natural expression of facial affection.
Playing "girlfriend and boyfriend" could indicate sexual abuse or could mean they hold hands and pretend to go to the pictures. An 11yo boy is likely to find either case embarrassing and not want it talked about. Any 11yo boy I know would be mortified if it were known that he played anything with a 6yo little sister.
You clearly have a 'concern' in your mind or you’d not have posted. So you need to follow your safeguarding procedure. Screaming "red flag" is a bit dramatic before very much is known; it merely demonstrates some people delight in jumping to the worst of conclusions ahead of the facts. Following procedure means calmly collecting facts before deciding on the appropriate action to take. Collecting facts means open question to both children and, possibly, the parents.
The appropriate action might be to do nothing, if you are satisfied there isn’t no risk. It can mean discussions with the parents, if you are satisfied no harm has been done, but there may be a need to ensure such games remain innocent. It can mean a call to your Children’s Social Care safeguarding hub if you suspect a clear and present risk.
Whether or not you involve the parents is entirely a matter of whether there is reason to believe a child would be at greater risk by doing so. (ie. could it lead to further harm or covering of evidence?)
I have some concern over this statement: "I know the family well and know it cannot be broached with them without lifelong repercussions." Could you please explain, as it may have some bearing on the matter.