Not sure if I understand your challenge. Could you elaborate and share your workbook in .tbwx format?
I assume the top table is your data source and the filter sheet is your desired result.
If so, I was able to get this to work as follows:
1) To get multiple brands on the same row, create a calculated field as follows (Concat Brand):
PREVIOUS_VALUE("") + IIF(INDEX()==1, "", ", ") + ATTR([Brand])
2) Place Brand on 'Detail' on the Marks card, and Tv Size and Concat Brand on the 'Rows' shelf.
3) On the 'Rows' shelf, modify the table calculation to use Specific Dimensions, check Tv Size and Brand, and repeat every Tv size.
4) To only get the last row of the concatenated list to appear, create a calculated field as follows (Last Only):
LAST() == 0
5) Place Last Only on the Filters card, filter for True, and modify the table calculation to use Specific Dimensions, and check Brand.
I hope this helps. I am not sure what version of Tableau you are using? I can upload a worksheet as an example if needed.