Transgender Support Group
This community is here to support and help the large population of transgender people. If you or anyone you know identify as transgender, this is the place to share your feelings and experiences and speak with others who are going through similar things in their lives.
All you can do is try to remind them, and talk to them. They will do their best, but they are going to make mistakes and often. Sad as it is.
I have taken the tact of looking at the pair of them like they are insane, every time they do that in front of people. Most people also look at them like they are nut cases, especially since I have been sporting a full beard for some time, and my gender marker on my DL is male, with my current legal (male) name. The rest of the family work hard to use the correct pronouns, and correct name - even to frowning at the two purposely wrong sister-in-laws and openly correcting them, in public - as loudly as they insisted on proclaiming the wrong name/pronouns.
My relatives have all heard and seen the horror stories of other Transgender people getting beaten up and killed, just for being themselves, so they are aware how dangerous it is for me - and how these two sister-in-laws are putting my life at risk, every time they pull this junk. Even my sister, who has been the slowest to come around, scolded them in public, saying, "Now, you two cut that out. You KNOW he's as much a guy as you're women!" (Which made my day, as the two of them gaped stupidly at her and everyone laughed at them.)
It takes time, for them to come to terms with it, and some of them may never do so - accept that, and keep working towards becoming yourself. As they see you becoming more and more comfortable as who you are, most will adapt and adjust - or fall by the wayside. Don't let it upset you - people enter and leave our lives all the time, for various reasons. We may not want to see them leave, but in the end, that is their choice. I like to think of it as them leaving to make room for someone else to enter my life.