Sister of my wife asked 500k baht to her Thai husband at the time of their wedding. The guy also had to pay for the wedding but most of it was recovered from the guests' donations (300 people).
When it was my turn to get married, the mom of my wife asked for 1M THB, my wife is more educated than her daughter (bachelor from well-known uni and MBA in UK, mostly paid by her family)
After negotiation, I agreed to pay 500K THB and show a total of 1M in cash that would later be given back to me after the wedding. Her mom just wanted to be proud in front of the guests and family to show that the farang husband was able to pay that much.
What I didn't know, is that after a year of being married, her mom sent me back the 500K and also gave us a few rais of rubber trees in the south that generate a monthly income of around 30-45K. Of course, it's all in the name of my wife but we used that asset to negotiate a loan with the bank to buy our house in the suburb of Bangkok. Her mom lives very modestly in her house in the south and I am super happy to take her on holidays with us, inviting her to nice restaurants, trips abroad and I even subscribed for very good health insurance in her name., which I am very happy to pay.
I guess it all depends on the intention of your wife's family. In my case, her mom wanted to make sure I would be able to make her daughter safe and happy and she didn't want us to struggle in life if we had to pay back a dowry.
Regarding her sister's husband, her mom bought them a house in a nice muu baan (after 3 years)