This study aims to explore the relationship between sibling number, birth order and maternal time investments in preschool children. Using the GSOEP and "Familien in Deutschland", we investigate maternal time investments in children aged 2-3 and 5-6. The dependent variables consist of two dimensions extracted from the mother's self-reported frequencies in various activities with children activities at home and outside activities. By fitting a random-effects and a fixed-effects model for two measurement points, we give answer to two questions: (a) To what extent do maternal time investments vary between children in families with different sibling compositions? (b) How do these investments change for the same child across time with the birth of a new sibling? The results show that there are quantitative and qualitative differences in time spent by mothers inside and outside the home. (a) There is an advantage for the firstborn with increasing sibship size, even when compared to an only child. Investments in middle and lastborn children get rather diluted with each additional sibling. The child's sex matters only for activities outside the home. (b) Children are more affected by the birth of a new sibling when they already have a younger sibling. The effect of time investments is positive for activities at home, but quite ambiguous for activities outside the home and varies with the child's sex and birth position. Regarding covariates, child care support leads to an increase in maternal time investments, whereas mother's working hours and household income have no impact.