It’s not quite the case that Protestants cannot receive Communion “under any circumstances” unless they become Catholic, but the exceptions that exist are quite extraordinary and do not include receiving merely for the sake of participation in a Catholic wedding Mass.
Here are the options you do have:
One: You and your fiancee could have a nuptial liturgy without a Mass. In this scenario no one would receive Communion at the wedding because Communion wouldn’t be offered. If you and your fiancee (and her family) would like a Mass in conjunction with your wedding, you could attend Mass either the evening before or the morning of the wedding.
Two: If you and your fiancee want to have a nuptial Mass, you can speak to your presider about the possibility of your family receiving a blessing from the priest instead of Communion. (Please don’t assume that this is going to be possible. Because it is a gesture that is not mandated by the liturgy, some priests do not allow it. To be safe and avoid embarrassment for your family, you should ask your presider if this is something he will allow.)
Three: Those who are not receiving Communion can be invited to make a spiritual communion. The guidelines from the USCCB on the participation of non-Catholics at Mass state:
All who are not receiving holy Communion are encouraged to express in their hearts a prayerful desire for unity with the Lord Jesus and with one another.
In explaining this to your family, you might make the point that, for Catholics, receiving Communion is a statement of faith in the Eucharist and a sign of sacramental unity among Catholics. For Protestants to receive the Catholic Eucharist is tantamount to misrepresenting their own beliefs and thus undermines their own integrity, which is why the priest your grandmother knows of was wrong to allow Protestants to receive Communion. That is why the Catholic Church ordinarily restricts its Communion to practicing Catholics who are in a state of grace.
Am I sinning by receiving Catholic Communion?