Can Women Pray Wearing a See-Through Sundress?
Question

I am religiously committed and when I am at home I try to wear modest clothes with my family, but the dress I was wearing shows the shape of the body in the sunlight only, but not otherwise. I used to pray in it for a long time, nearly one year, then they told me that my prayers were invalid and would not be accepted, because it was not permissible to pray in this dress. How should I make up my past prayers? Is it permissible for me to set aside the time between ‘Asr and Maghrib to make up these prayers, because that is the only free time I have in the day?.
Praise be to Allah.The Obligation to Fully Cover the `Awrah in Prayer
It is obligatory to cover the `Awrah when praying, with something that does not show the colour of the body.
The `Awrah of the woman while praying is all of her body except the face and hands .
Is a See-Through Sundress Allowed for Prayer?
If the dress is light and shows the colour of the skin, white or red, underneath it, then it is not regarded as covering and it is not acceptable to pray in it.
If it was light but did not show the colour of the skin, there is nothing wrong with praying in it. The same applies if it does not show what is under it except when you stand in the sun: if the prayer was not offered in a sunny place, then it does not matter, because the prayer was offered when you were covered.
This only indicates that the dress is light, not that it is see-through and shows the colour of what is under it.
Ibn Qudamah (may Allah have mercy on him) said: What is required is to wear something that conceals the colour of the skin. If it is light and shows the colour of the skin underneath it, and one can tell whether the skin is white or red, then it is not permissible to pray in it, because it does not cover properly. If it conceals the colour but shows the shape, it is permissible to pray in it, because this is something that cannot be avoided. (End quote from Al-Mughni, 1/337)
An-Nawawi (may Allah have mercy on him) said in Al-Majmu` (3/176):
Our companions said: It is essential to cover with something that will prevent the colour of the skin from being seen. It is not sufficient to wear a thin garment through which it may be seen whether the skin is dark or light. (End quote)
The Shafi`is clearly stated that if (a garment) shows what is underneath it when there is sunshine or fire, and the `Awrah can be seen in that case but not otherwise, then this does not matter. Similarly, it does not matter if what is underneath it can be seen when staring at it or looking at it from close up. (See: Hashiyat Tuhfat Al-Muhtaj, 2/112)
Based on that, your past prayers were valid and you do not have to repeat them.
For in-depth explanations, see the answers listed here:
Can Women Pray While Wearing Pants?
Is praying in pants invalid?
Can Muslim Women Wear Pants?
And Allah knows best.