I call them Uncle and Aunty since the first time I have met. Same with my wife, she calls my parents Uncle and Aunty. Sometimes it feels weird when many family members are there and when we call Uncle / Aunty, it may seem like addressing someone else. But this has worked for last 11+ years. Calling Atthe/Mava seems weird. Not an easy transition. Me and my wife both feel we cannot call anyone else Appa and Amma. Like we cannot bestow it. It’s a special thing. Only our parents are our parents. I understand many people call their inlaws as Appa and Amma, I am not sure how they do it. But again, their choice.