There is someone very close to me who does this - like you, she is aware of it and realises that it's not good, but can't really help it. We have just come to an agreement that when she does interrupt me I gently stop her and tell her, "Hey, you just interrupted me." Usually she will apologise and let me finish after that, and the conversation goes on as normal.
Initially it was a bit awkward - we both got a bit frustrated and irritated by it, which is pretty normal really: overall the whole situation is a bit belittling and embarrassing. I don't want to feel like I was telling her off, and she didn't really like being told that she'd done something wrong; both pretty normal human reactions. But over time it's just become a thing that we do. I've come to terms with the fact that when she gets excited she may try and interrupt me, and she accepts that my opinion is important and if I cut her off because she interrupted me then it's justified.
Basically, if you adopt this practice and are all capable of being adults about the situation, it should be fine, barring some teething issues as you get used to it. It becomes more difficult when we're with friends or acquaintances and she interrupts me then: usually I won't say anything because anyone who doesn't understand the situation may thing that I'm overbearing or rude, and we have yet to find a workaround for that. But with family, if you're all on the same page, I would assume that it's something you can work together on.
NB: I don't wish to post this answer as a solution instead of any others, but more suitably in conjunction with them. However, while I agree that there are many things you can do alone to solve the problem, the best way to tackle a problem is together, with the help of people you are close to.