Mine are 8 and 10.
Myself, DH, My Mum and Step Dad Sister and Brother in Law kiss my kids on the lips.
My brother and sister in law prefer to kiss them on the cheek.
They prefer not to kiss DH family at all so don't.
If they don't feel like giving family a kiss they won't. It's just something they have always done.
My mum sometimes kisses me on the lips and I kissed my grandparents on the lips until they passed away.
My friend has said she's never kissed her boys on the lips.
Mine are girls not that it makes a difference.