From what I understand, Si Dian Jin is giving to "pin" down the daughter-in-law because the mother in law is supposed to "pin"/wear the jewellery on her. That's why your mum says your sis' future in-law cannot bully. But that's the custom. If in future really bully, your sis also cannot do anything.
Is your sis' future in-laws unhappy with the union? Do they feel that it is too expensive? Or do they feel that since your sis is marrying "in", she should follow their "law"? Or are they tight financially? Is your sis expected to meet any customs of their teochew dialect? Have your future bro-in-law checked with his brother that his wife only received a gold bracelet? Or the mother in law cannot remember what she gave?
Your future bro-in-law should cool down. If he just ROM and fly off for honeymoon, your sis is going to have a hard time with your in-laws.
Cool down, try to find the root of the problem. It seems like there are deeper issues than this.
Are the in-laws unhappy that there is no banquet? No banquet, so they also dont't want to spend so much on their daughter in law?