How do you get your husband to ask his mother or other siblings what are the plans of hers (my mother in law) when she is needing care?

Yes, you are the one they are counting on since she lives with you now.

Nip that in the bud, as Barney Fife said. And do it now before they all start ganging up on you when the time comes.

I'm sure your mil's plans (and the rest of her family thinks so, too!) that YOU will be the one to provide the caregiving, when the time comes.

It's not too late to derail that train!

Is he afraid of his mother or afraid of the subject of her getting older and needing help?

I assume that you have told your husband that you will not care for his mother?

Maybe some marriage counseling would help?

You can ask her --- on a good day in an easy going manner. You know ____, I've been a caregiver in a number of situations. Needless to say things are great right now, but if you needed more help, do you have any paperwork we should know about? Health care proxy? Does someone know where your important papers are - like a will and any insurance.
Play up a messy situation (real or hypothetical) so she sees the importance of this.