I can only carry so much

Len Rose doesn't want to have any more kids after the twins for various reasons, so by definition, StS!Monty would have to be a cousin, or completely unrelated to the Roses or Lelands; I imagine Sonic's attitude towards him would be largely unchanged ("largely" because he's got mild father issues).

Amy roped Sonic into carrying both her and Monty because why not. (Admittedly, Sonic's weight limit may actually be higher than this, assuming Elise weighs more than my and Monty put together)