Madri's beauty is not described I believe.. or I dont remember.. so cant comment on Kunti getting jealous of that.. same with Pandu's partiality.. Kunti was not really jealous of Madri but she felt cheated by her.. Kunti can not bear more children as she was done with her limit of 4 men in her life (keeping suryadev as a secret).. she was afraid if Madri will again invoke twin gods to get 2 more children and surpass her in number of sons that too with the help of her Mantra..
but Madri would/should not invoke twin gods again because that would have made 5 men in her life.. Kunti might have clarified or restricted Madri instead of denying..