My sister and my baby daddy/ex are in love... How do I tell my our 2yr Old Boy?

Here's the situation... Me and my ex/baby daddy have been split up for 1 year and some months.. he recently started coming around more to see his son which is 2... but for the last month my sister and him have developed feelings for each other.. they came to me and im ok with them being together because i will support both of them and what makes them happy.. the only issue we all have is how to explain to our 2yr that his aunt is dating daddy...

if anyone has ever been in a situation like this or has any advice or input feel free to comment... I KNOW YOU DONT EXPLAIN TO HIM AT THIS AGE.. BTW... BUT THE DAY WILL COME... KINDA TRYING TO FIGURE OUT ANSWERS TO THIS, THE SOONER THE BETTER.... ALSO MY FAMILY THINK THAT THIS IS WRONG AND NO ONE  SHOULD CONSIDER THIS SITUATION OK?

IS THIS TRUE?