If she doesn’t understand, you could use this as an opportunity to educate her. Explain what being genderfluid and gay means, and tell her about your personal experiences. My mom didn’t understand being non-binary at first, but she supports me now after I explained what it was and how I feel.
Since you’re engaged, you could also wait until you can move in with them, for a safe option in worst case scenario she doesn’t support.
#Warning homophobic family⚠
unborn yew
split ice
Try to educate her about what being gay and genderfluid is first(pride month is good time for this), and then try to find out how she'd react to being gay/genderfluid without actually telling her you are.
If you find out you're not going to be accepted then don't tell her though.
Have a backup plan for when you do tell though, somewhere to crash at and ample money.