I looked at this issue in October 2010 and, unless something has changed since then (which is unlikely) there is no univeral legislation on the matter, and your sister-in-law is permitted to go up for a blessing, unless you know that your priest or diocese does not permit this practice. You could set you mind at ease by phoning the priest, parish office or Cathedral for advice.
Now, answering YTC’s post.
**It is the opinion of the Congregation for Divine Worship and the Discipline of the Sacraments in the Vatican, as in this document:
forums.catholic-questions.org/showthread.php?t=543513**
that it is improper for anyone not receiving communion to approach the altar in the communion line to receive a blessing. Non-Catholics (and also Catholics who are not properly disposed) may not receive communion at all, so it is improper for them in all cases to approach the altar in the communion line, even to receive a blessing. The Congregation deems it inappropriate to attempt to receive a blessing in the communion line because the communion line is solely to receive communion, and
everyone receives a blessing at the end of mass anyways right before it ends, and you don’t get any “extras” by being blessed twice.
Please be more careful when invoking such an authority as the Congregation for Divine Worship and Discipline of the Sacraments (CDWDS). Your link is not to a document from the Congregation, but to another thread in this forum. In that thread there is a link to a unofficial reply to a private letter, which begins…
This Congregation for Divine Worship and the Disciple of the Sacraments acknowledges receipt of your kind letter of 13 August, 2008 and would like to thank you for your interest and suggestions. This matter is presently under the attentive study of the Congregation. For the present, therefore, this Dicastery wishes to limit itself to the following observations:
The observations which follow are close to your summary, however your summary is, I think, just your own words, and not from any Church document.
When I looked at this in October 2010 the best information was available
at Zenit. The whole document (which is really two documents), includes some opinions similar to your own, but it goes back and forth on the subject, without communicating any decision (except that there is no decision).
I don’t know if there has been any ruling since then. If not, then there is still no prohibition on the practice, and a Bishop could give guidance, or an individual priest, so long as he was not in conflict with his bishop.
One para in the document…
Because of this, some dioceses have taken a prudent wait-and-see attitude regarding these blessings. For example, the liturgy office of the Archdiocese of Atlanta, while reiterating that “the Archdiocese has no policy prohibiting the use of blessings at the time of Holy Communion,” prudently suggested to pastors that it "may be appropriate to avoid promoting the practice until a more definitive judgment regarding its value in the liturgical celebration can be obtained."
That is hardly a condemnation!
You are not wrong, except in attributing the statement to the CDWDS, and hence implying that the matter is settled.