Dottie, The others said it for me. You shouldn't do it & it is best.
In a 'nice' & gentle way tell your mom 'I know that would make you both happy but I just can't mom. Since uncle ___ came it's been hard. I want you both to have all you need but if ___ comes, she'll soon need help (maybe mom would say 'but she is just fine') and I can't always help 3 of you. Even 2 is hard.' Don't make it sound like "That Would Be Impossible" cuz she isn't thinking about your health & ability to care for 4 ppl ("four" includes you).