It isn’t quite clear what your question is. Is your worry about telling them that they may not receive communion in a Catholic church?
How does your pastor or the priest who will be celebrating the first communion mass feel about this? Is he someone you could ask to make sure to make an announcement explaining the “only Catholics may receive communion” policy at the ceremony? That way, it wouldn’t be you telling her, not personal, it would be the church.
Of course if your priests don’t really agree with this Catholic teaching and don’t really care whether non Catholics receive communion, which is the case in some parishes and dioceses, then you have to ask yourself whether you want to try to stop your mother in law from receiving communion even if your priests don’t care to do so and even if other non Catholic relatives will be receiving.
Some Episcopalians do have a very Catholic like belief in the real presence, and some people interpret the rules about intercommunion in exceptional circumstances to include weddings and first communions. This is closer to being within official guidelines than just not caring who takes communion. It is possible to make an argument for it, although I suspect the apologists here would not accept it.
If you can’t count on the priest to make such an announcement you can say when you invite her that the official Catholic rule is that only Catholics can take communion, If she objects or wants to know why you can say because we think it is a sign of complete unity which we think includes agreeing with all Catholic doctrines and being in communion with the Pope.
If she says something like “Communion is for all Christians” you can tell her that is one of the things we disagree about but you would still like her to attend. If she tells you that if she attends she will receive communion, I myself would not disinvite her. I would say , “That’s up to you, I am just telling you what we believe.” Once you tell her, you have done what you could. It isn’t your sin if she receives. I don’t think you need to feel that this is a great sacrilege against Our Lord that you have to prevent at all costs.