your parents sound toxic. have you considered taking a break from them and seeing if you feel emotionally healthier?

It's not about the money - you and your sister are adults and should not need your parents to provide financial help. However, they are also adults and if they ask for any more financial help - say no. If they owe back taxes - well, that is on them.

Be emotionally supportive of your sister. If you are financially flush and want to help financially, that is lovely, but do it because you genuinely want to, not from a sense of guilt. Healthy relationships are based on mutual respect and affection, not guilt and shame.