If both your mother and sister have burning sensation in the affected areas, then it cannot be simple tanning of the skin. There is some element of a reaction in the skin. It could be photosensitivity leading to both pigmentation and burning. It is likely to be more in summers because (1) the intensity of light in summer is more, and (2) they should shed off as many clothes as is possible to escape heat and hence expose greater areas for longer periods to the light (to act on skin). Once it is established that they have photodermatitis or photomelanosis, then the photosensitizer should be looked for and if possible eliminated. Else one will have to take photoprotective measures.