You are asking to do a THANKLESS job. It will get old fast. You will have all the responsibility and none of the authority. You will be caught in the middle of THEIR parenting.
You will have two bosses and they will not agree with any of the parenting of the kids.
Your life will get messed up and none of it will be because of what you have done.
After YOUR divorce you will feel an awful loss because of the relationship you developed with the kids that aren’t even yours and YOU will not get visitation with them either as they have parents of their own.
Need more reasons?
Been there done that.
Let him visit you in YOUR home until the youngest turns 18. It will be much easier

No it is to hard to try and raise someone else’s kids and the kids are torn between mom and dad.