Yeah well, it all revolves around a big series of what if. What if Muntz had left behind a wife when he took off for Paradise Falls all those years ago? What if it was the elderly French lady that we see in Ratatouille?
Why I think they would make a good match; they're both the most heavily armed old people I have ever seen (he has swords, gun and airplanes, she has gun and anti-gas mask). They should approximately be of an age (though we don't know when Ratatouille takes place). You don't want to be the intruder who steps in their homes (or in Muntz's case, in the blimp). They both like good kitchen! And besides the lady's gas mask looks like it might come right from world war II, as do Muntz's planes
Hence my theory - she's Muntz's wife, of French origins, who moved back in France after her husband did not return for so long.
And just to add more craziness, I came up with another relation! Let's say that the French lady was with child when Muntz left for Paradise Falls. Now let's say that the child was a daughter. The mother might have returned in France only after their daughter was grown up - so what if the daughter stayed in the states? What if she married a certain Mr. Philips and became the mother of Sid? Muntz and Sid's faces are a bit similar as far as shape goes - and we see a deer head in Sid's house, and most logical assumption would be that it's either a present or a hunting trophy of Sid's father. But the only time we see Sid's father he's sleeping in front of the tv, so hey, couldn't it be Sid's mother who goes out hunting, and takes the love for guns from both parents?