Haven't recieved thank you for wedding gift.

My niece was married Labor day weekend in a distant state. We couldn't attend but we sent a generous check with our daughter who flew out for the celebration. Last weekend I saw a thank you card from them on my brothers fridge (his wife attended). We have not received one. What should I do? It was a lot of money for us, and I am hurt our gift has not been acknowledged. Our bank statement showed the check had been cashed 6 weeks ago.