The Church has a directive on this. The General Instruction of the Roman Missal states:
. . . as circumstances allow, they may sit or kneel while the period of sacred silence after Communion is observed (43).
In 2003, the Congregation for Divine Worship and the Discipline of the Sacraments said the following about the above (GIRM #43):
The [intention] is that that the prescription of the Institutio Generalis Missalis Romani, no. 43, is intended, on one hand, to ensure within broad limits a certain uniformity of posture within the congregation for the various parts of the celebration of the Holy Mass, and on the other, to not regulate posture rigidly in such a way that those who wish to kneel or sit would no longer be free (Prot. N. 855/03/L).
What this means is that you are free to sit or kneel after communion. I normally advise people who can to show some reverence by kneeling for at least a while before sitting. However, the Church is saying you are free. There should be uniformity, yes, but you’re not rigidly required to do either. Therefore, it is not sinful at all to sit after receiving communion.
