Husband's ex-wife always takes her children back to see her mother-in-law

This is her right, I suggest you don't object. This shows that you are very generous.

What you have to do now is to treat the children of your husband's ex-wife as your own children, and don't reject them. Next, my husband will love you more. This is because your generous love and honest behavior have won your husband's respect for you!

Wish you success!