สะใภ้//sàphái simply means ‘female in-law’.
Wife of younger brother = น้องสะใภ้//Nórng sàphái (regardless of her age – even if she is older than you, she is still regarded as such.)
Wife of older uncle = ป้าสะใภ้//Pâh sàphái
Wife of father’s younger brother = อาสะใภ้//Ah sàphái
What about ‘male in-law’?
Husband of older sister = พี่เขย//Phêe khoĕi
Husband of older aunt = ลุงเขย//Lung khoĕi
Husband of mother’s younger brother = น้าเขย//Náh khoĕi
Ok speaking of relative terms, let’s see below for a really basic Thai family tree: