It's weird to have this kind of attitude when you are married. The whole point of marriage is that two people's earning power and resources being pooled together as a partnership is more stable than each individual being self-reliant.
If you wanted to be fully independent you should never have married, and should untangle your finances and divorce as soon as possible. You can keep the actual relationship going if you wish, but as two individuals unbeholden to one another.
Within a marriage, it's supposed to be a partnership. Sometimes one individual is bringing in more finances than the other, sometimes its the other way around, and both people work for the good of the partnership and often whoever is doing less by way of bringing in finances from outside is doing more in terms of household tasks, but in theory as you love one another neither of you is being taken advantage of, and it all balances out. Any debts are likewise shared (and neither party should incur serious debt without the other's agreement). If it's not like this then your marriage is in trouble anyway, obviously.
In such a context no one is beholden to anyone and if an inheritance is recieved it is the property of the partnership and isn't owed to anyone. It wasn't a debt, it was a gift, and it was weird to refuse it in the way you did. The credit card debt obviously was a debt, and there was capital available to repay it, and it was sensible to repay it.
An entirely separate issue would be if your MIL was trying to use the fact of this financial transfer to be in any way controlling of you and DH's decisions but that would be a derail. However if she wasn't trying to control you then it's perfectly normal for parents of adult children to make large lum-sum gifts as part of planning for inheritance tax (if you make a lump sum gift and then manage to survive a further 7 years then that lump sum doesn't get counted for inheritance tax, assuming you are in the UK).