If you want genuine help, ask on a place that will actually want to help you maintain a relationship with them, like reddit's asktransgender here people will just help you to completely ruin your relationship in promotion of their hatred.
And the reality is that you have to decide, do you want to keep your hate against trans people, or are you willing to genuinely accept your new brother in law and learn. Trans people can spot performative support from miles away, faking it won't work. But if you genuinely are willing to show effort to accept them as they are, they will help you and guide you through it.
If not, well, enjoy being excluded from their life very quickly.
I will give you a starting point on learning to accept checklis:
> Are people really happy identifying as another gender when they wouldn't look like the other gender?
Being transgender isn't a choice, it's a hand you are dealt and have to deal with, and the best treatment for it is coming out and acceptance, for many that also includes all sort of medical interventions but it's not necessary.
If you will keep thinking that it's a choice, or that they are not aware of the presentation issue, but persevere despite it as otherwise the life is just not worth living... Well, that's the performative support trans people just do not fall for, not for any length of time.
You can also DM me if you want to speak more openly in private.