It is forbidden by the Catholic Church for Catholics to receive communion outside the Catholic Church except where She has expressly allowed it . . such as in EO communions because of what communion is and what it signifies. To do so is hypocrisy on the part of the one doing it.
Holding the Eucharist to be the sign of unity of faith and practice, the Catholic Church naturally considers it hypocrisy for one not having the same faith and practice to receive Communion. Therefore she forbids Catholics to be receiving Holy Communion outside the Catholic Church. (Incidentally some Protestant Communities teach their members the same thing.)
Here is the canon that affects this:
Can. 844 §2 - Whenever necessity requires or a genuine spiritual advantage commends it, and provided the danger of error or indifferentism is avoided, Christ's faithful for whom it is physically or morally impossible to approach a Catholic minister, may lawfully receive the sacraments of penance, the Eucharist and anointing of the sick from non-Catholic ministers in whose Churches these sacraments are valid.
In the Eccumenical Directory,(1993 Directory for the Application of Principles and Norms on Eceumenism) we see this:
123. Whenever necessity requires or a genuine spiritual advantage suggests, and provided that the danger of error or indifferentism is avoided, it is lawful for any Catholic for whom it is physically or morally impossible to approach a Catholic minister, to receive the sacraments of penance, Eucharist and anointing of the sick from a minister of an Eastern Church.
132. On the basis of the Catholic doctrine concerning the sacraments and their validity, a Catholic who finds himself or herself in the circumstances mentioned above (nn. 130 and 131) may ask for these sacraments only from a minister in whose Church these sacraments are valid or from one who is known to be validly ordained according to the Catholic teaching on ordination.
You ask if it is a sin to partake in communion in a church service where the sacrament of the Eucharist is not valid . .
What you are actually aksing is, it is a sin to disobey the Church?
if it is . . then it is a sin to partake of communion in churches who do not have a valid sacrament of the Eucharist and according to the instructions and limitations imposed by thr Church upon the faithful.
Peace in Him!