Olivia is referred to as Sir Toby's "Niece" but his precise relationship isn't clear as she refers to him as "cousin" which is more general
Sir Toby Belch's name refers to over-indulgence and in most of his scenes he is drunk or drinking
While Sir Toby relies on Olivia's goodwill to stay in her household, he exploits Sir Andrew by encouraging him to woo the countess
Sir Toby marries Maria at the end of the play
It has been suggested that in the characterisation of ST, Shakespeare was trying to recreate his most successful comic conception, the fat and disreputable knight of the Henry IV plays - Falstaff
Although Sir Toby is presented as Jolly, it may be a case of over-compensation for a man who has lost a nephew (Olivia's dead brother)
Sometimes Sir Toby has been played as a man having one last fling before settling into a more respectable old age, for example, in Feste's song the references at time passing in the drinking scene
If Sir Toby is having a last fling before old age, his marriage with Maria is part of the "last chance" mood
"Confine? I'll confine myself no finer than I am"
Confinement, modesty and order are antipathetic to Toby's excessive and disorderly conduct, words and language are not in his control when drunk, rejects authority of clock-time and by extension the limitation of time itself, his swaggering excesses are punished at the end of the play when Sebastian wounds him leaving him ironically to berate drunkenness