Quoting Sabrina Norvitz:" Physical Side Effect Of Miscarriage: There are many types of miscarriage and some of them can have serious ... [snip!] ... time can be longer for some women. You need to give your body the time to get healthy and healed to have a safe pregnancy."
You just trumped yourself, I hope you know.
Having had prior miscarriages doesn't mean your body is weak, or that it's more prone to infection than someone that hasn't had a miscarriage. It also doesn't mean that you are high risk. Or that you aren't able to carry a baby to term. And the chances of one (or two) miscarriages making you completely infertile are SO incredibly slim. Something would have had to have gone seriously wrong in the miscarriage process for that to happen.
Even someone that had a stillbirth (which, FYI, if your sister lost a baby at 28 weeks it was a stillbirth, NOT a miscarriage).. that has no indication on whether or not you can carry full term.