WRT heritability, if your wife-to-be is genuinely an identical twin then the chances of her birthing twins is as much as for any woman. Her being a twin makes no difference.
What could make a difference is that, genetically, her children by you, and her sister’s children by that other husband, would be half-siblings. If you have a culture where cousins or even second cousins marry then this could a concern many years down the line.
It’s also not always that easy to determine whether a twin is actually an identical twin, sharing identical genes, or a fraternal twin. Fraternal twins are an inherited trait. If the twins are of different genders then it’s pretty easy to say that they were fraternal twins.
If two placentas came out, then they were almost definitely fraternal twins and likewise one placenta usually means identical twins. However, there have been cases where fraternal twins have shared a placenta and cases where identical twins have had individual placentas - it’s all due to when they separated. Absent very obvious markers like gender or blood type you can’t be certain whether a twin is fraternal or identical.