I’m a Catholic (‘womb to tomb’) who has had a lot of experience witnessing my Catholic faith to our separated brethren. Though we obviously disagreed with the usual doctrinal points they always bring up, they trusted me because I am a “Catholic who is also a good Christian” (to which I was always graced by the Lord with the strength not to say, “Duhh!”). Because of their our mutual trust - springing from a faith based friendship - they asked me the same questions about their Catholic parents and in laws.
** I’ll give you the same advice I gave them, use it or not with the Lord’s guidance. The 4th Commandment is to “Honor your father and mother”. In ‘ecumenical’ (aka religion mixed) families the subject that divides us ALWAYS comes up. Especially at holiday gatherings, it seems. Both ‘sides’ are trying to convert (‘coerce’?) each other. In our families we all know each others buttons and are both quick and adept at pushing them to the point where one matriarch or other will - after and uncomfortable silence - adroitly change the subject.**
If alienation occurs (or, more often, ‘when’!), it’s usually not mended til the granchildren appear on the scene.
You were right in thinking that you MAY never see your in-laws’ conversion this side of eternity. Treat your in-laws the way God has commanded. Treat them as a loving son-in-law first and foremost - not as a Catholic “invader” into their spiritual lives.
**Follow St. Paul’s injunction to always be ready to show “the reason” for your belief, but avoid attacking his. Stress the common faith you share (summarized in the Nicene creed). Rather than seeking ways, times and places to challenge his unbelief, find ways to SHOW HOW your faith makes you live: Protestant Evangelicals are often hungry for the Eucharist they don’t consciously believe in. When they see Catholics make the sign of the cross when praying (even in restaurants), mutely genuflecting in front of the Blessed Sacrament, blessing ourselves with holy water, praying the Rosary, it raises questions in them. Sometimes they express this by ‘attacking’ our actions and accusing us of unbelief- etc, etc. **But even more often they witness it and wonder. There may come a moment when they ask why we do these things - and because they really want to know. These are the grace prompted moments for authentic Catholic evangelism.
**Don’t be upset if they ask some other Catholic they know instead of you. Scatter the seed, let God tell you if you’re to water and tend. **
In Christ