It was the custom of the times for the bride to wear a covering, a veil, over her face. Jacob's mother, Rebekah, had veiled her face when being presented to Isaac.
For she had said unto the servant, What man is this that walketh in
the field to meet us? And the servant had said, It is my master:
therefore she took a vail, and covered herself. (Genesis 24:65, KJV)
No conversation between Jacob and either Rachel or Leah is recorded during the wedding. The fact that Leah is called "tender eyed" implies that she was shy.
Leah was tender eyed; but Rachel was beautiful and well favored.
(Genesis 29:17, KJV)
The lexical entry for this word actually indicates "soft of words" and "timid" as possible meanings for this. So Leah would have been quiet by nature, and would not have found it difficult to maintain her silence with Jacob. Furthermore, she would likely have been frightened to think of what might happen if he discovered the truth.
Though Laban claims it was the custom of the time for the eldest daughter to marry first, we do not have proof for this, and few other examples would help us establish the point one way or the other. Had it been the custom, it seems Jacob might have been more wary of the possibility that Laban could pull a fast one on him like this. Only one sister (Rebekah) is on record for Laban himself, so he would not have seen an example of marriage order like this within his own home.
The final point to remember is that Laban brought Leah to Jacob when it was already evening, i.e. it was dark.
And it came to pass in the evening, that he took Leah his daughter,
and brought her to him; and he went in unto her. (Genesis 29:23, KJV)
That "evening" might also be translated as "night." If the veil were not enough to hide her, darkness was there to assist.