Possible residential floor area

Three apartments per residential unit are permitted (each half of a semi-detached house counts).

This assumes that one semi-detached house is built per plot. So even a plot with two semi-detached houses will have a maximum of three apartments. Your idea is already almost a classic here, discussed in detail, among others, in as well as - I just unsuccessfully searched for at least one more from 2017 or 2018 (?).

Your project is not that simple.
No idea if you brothers are single, stay that way, or have families.
But if you build a two-story building where one side, ground floor and upper floor, is a residential unit on, for example, a 9x9 area (so a typical semi-detached house) for a family of four and attached to it a two-family house with one residential unit on the ground floor (parents) as well as the upper floor for a couple on an area of 108 sqm, then the floor area ratio is already reached with 380 (189 x 2, simplified calculation). Of the permitted 270 sqm, 189 are deducted, leaving 81 sqm for paths and ancillary facilities. That is not much.

What does the development plan say regarding § 19 Land Use Ordinance?