Thanks for posting!I do not have a great deal of information regarding any of the siblings for your Sebastian Meng, JR. except for my own g-g-g-grandmother, Margaretha Meng Holzer.
I do know that she was married in Trimmis and that she came with her husband Johannes Holzer and their son John on the same ship as the Mengs to arrive in New Orleans on 15 Oct 1855.They did settle in Green Bay, WI, where Johannes died while walking to Peshtigo Wi, looking for work in the sawmills when he could find no work as a chemist in Green Bay.His burial site was lost in the Peshtigo Fire.
My information is largely collected from the labors of other better researchers!
As far as your ancestor having an identical name with her mother-in-law, I wonder if the names were only portions of full baptismal names.I have found that our ancestor Barbara Wilhelm Meng was apparently CATHARINA ANNA BARBARA Wilhelm Meng.The two women could have been related distantly and shared some names. (I have found many female ancestors had more middle names or first names, but only ever went by one of them, even on some legal documents.)
Best of luck with your research!If I can help more, let me know. Julie