You're dating your sister's best friend who is 10+ years older than you? I don't think the problem is her using the word 'mom'.
Idk y shes doing it. I think it kind of weird for her to do that. Almost like she wants to embarrass you. U should talk to her and tell her how u feel about it.
I don't think your sister is being disrespectful, especially if she accepts being called that by your sister.<br />
If they're ok with it, then don't let it bother you.
Feelings...feelings...feelings...in all you three make all these differences.