There is no problem if one is sneezing unintentional during prayer.
As for yawning :
It is reported from Abu Huraira (RA) that the Prophet (salla Allahu alahi wa sallam ) said: “Yawning comes from the Shaytan, so if one of you feels the urge to yawn, let him suppress it as much as he can, for if one of you says ‘ha’ (the sound made when yawning), the Shaytaan laughs.” ( Bukhary and Muslim)
However, if you could not fight this at least place your hand over your mouth while yawning .