This study concerns simultaneous bilingual acquisition (2L1) of Swedish-Brazilian children growing up in mixed-lingual families in Stockholm, with Swedish as their dominant language. Earlier studies on this language combination were not found. Not even were there any studies considering 2L1 children of the same age group as our main subjects (Anna 7;7,3–9;1,30, Maria 6;1,16–6;11,11). An analysis of their acquisition of Brazilian Portuguese (BP) as a weaker language (WL) was carried out in a Generative Grammar approach, mainly through the selective theory of language acquisition. The corpus consists of interviews with 2L1 children in a semi-longitudinal registration of their production. The focus of this analysis lies on the observation of three domains of BP grammar that differ morpho-syntactically from Swedish: verb inflection; VP as minimal responses; NP number and gender agreement. Three main research questions were formulated: 1) Are the simple and robust structures, provided by domestic input enough for triggering the functional categories (FC) of their WL? 2) If the FCs are activated, do they develop in the same sequence as a WL as they would in BPL1? 3) If the 2L1 children show any deviations in acquiring the grammar of their WL, is it possible to distinguish any influence from Swedish? To answer these questions a contrastive study was carried out, comparing the acquisition of BPWL with studies on 2L1 and BPL1 acquisition. The results show that the domestic input is enough for triggering the grammar of the WL, and that it was triggered and developed through a similar procedure to that of BPL1, although delayed. Contact with BPL1 input in Brazil was necessary to activate the children’s oral production. When using VPs for minimal responses it requires more than domestic input, and the influence of Swedish was reflected in the subjects’ use of sim ‘yes’ instead of VPs, as in this case grammar enters the domain of discourse at the syntax/pragmatics interface.