I have read somewhere (forgot the source since it was some time ago) that the Adepta Sororitas menstruate normally. If this is true and my memory does not fail me, can they have children? Because it would be quite near impossible with all their enhancement and battle wounds.
According to a footnote in "Cain Last Stand" Adeptas Sororitas don't take vows of celibacy (and if you believe even part of the fanfic you can find on 1D4Chan they have sex quite often...). So yes, there is no technical reason why they wouldn't be allowed to have children.
You have to understand that Imperial Cult is not as united and monolithic like for example Catholicism (or even Christianity) - there are a thousand different religions with different rules, each one has just one thing in common - the Emperor is the key figure.
Saying all that there is very unlikely that a sister (especially from the militant order) would decide to have a child: They are all fanatically devoted to the Emperor - they call themselves "Daughters of the Emperor" - (as opposed to the "Brides of the Emperor" during the Age of Apostasy) and being pregnant would (in their own eyes) reduce their usefulness, which is of course sin.
But in the non-militant ordos, as long as it wouldn't affect her duties...