Why were the twins enslaved?

From the text:

“Our father survived his misfortunes only a month, our mother soon followed him, and we were alone in the world. Our parents could have made themselves comfortable by exhibiting us as a show, and they had many and large offers; but the thought revolted their pride, and they said they would starve and die first. But what they wouldn’t consent to do we had to do without the formality of consent. We were seized for the debts occasioned by their illness and their funerals, and placed among the attractions of a cheap museum in Berlin to earn the liquidation money. It took us two years to get out of that slavery.