Miscarriages

I'm sure some of you ladies know alot more about this than me but I'm curious my sister in law got pregnant in high school had a perfect little boy he's almost 4 I believe we'll they've been trying for another one for awhile. beginning of this year I think they had a miscarriage. 😢 She got pregnant again in August sometime, and she just lost another one.(can't seen to make it past 8 weeks) How can you have a baby and then all of a sudden not have no more? What could be wrong with her? The grandparents of this little boy say that the father should DNA test him they don't think he's the father. (This boy just looks like his mama) I feel awful for her they're both takeig the loss pretty hard. But I was still curious anyone had similar problems??