moms wedding dress?

So I will be 15 weeks pregnant tomorrow and am currently trying to plan my wedding. Me and my dad have been talking about what dress I'm gonna wear and my mom still has her wedding dress from 18 years ago and my dad asked me if I'd like to wear her dress. Well my mom was very little when she got married and I'm much bigger than she was then. My aunt, which is my dad's SIL has her wedding dress still and it's bigger than my moms and I think would flatter my body a lot more than my mothers, even after doing alterations. So I mentioned something to my dad about it and he spoke to my mother which he came back and told me it had made her upset and that she was crying. I feel as if I let her down but at the same time, it is my wedding and I will be the one wearing the dress. Not sure what to do..any other one of you ladies been through anything like this and can spread me some advice??