Need advice, I just found out that my husband might have a daughter with a previous marriage and he didn't know either what would you suggest on this?

Get a DNA test done to confirm whether it actually is his and then you’ll need to navigate with the mum what she’s told the child about her dad, whether she lied and said he died etc.

Then I would find a really good child psychologist and arrange family therapy sessions to introduce your husband to the child and then later you and any kids you have together, in order to make the 6yo transition into discovering her father and new siblings easier.

This can happen - some fathers don’t even know if the woman doesn’t tell the father or may not know who the father is. All you can do is support him and work through it together , I know it must be difficult ❤️