I'm not sure if this is part of her reasoning or how common this is, but I did read that sometimes if babies are conceived too close together they could be at risk for preterm labor. I remembered this because DH was 2 months early and is 9 months younger than his sister.
This is from the March of Dimes website:
"Short time period between pregnancies [One study found that an interval of less than 18 months between birth and the beginning of the next pregnancy increased the risk of preterm labor, though the greatest risk was with intervals shorter than 6 months. A woman should discuss with her provider the best pregnancy spacing for her.]"