I am not sure which IPUMS dataset you are referring to, but the answer is broadly applicable to all IPUMS data. You may be able to identify the sex of the respondent’s co-resident siblings assuming that the data lists basic demographic information about all persons in the household (this is generally true of data available through IPUMS). However, without a specific question about siblings outside of the household , you cannot know if the person has siblings or their sex.
Assuming you are interested in the sex of co-resident siblings only, you could look at value pairings of the variable that reports the relationship to a reference person (usually called RELATE in IPUMS datasets) that indicate siblings (e.g. if two persons are both “child of householder” then it seems reasonable to infer that they are siblings) and then append the sex of each sibling a person has as an additional variable on their record. Note that the children of householder example is fairly straightforward (another straightforward example is siblings of the householder). However, some relationship pairings that could denote siblings may not be as straightforward (e.g., a parent of the householder and an aunt of the householder may or may not be siblings; the aunt could be the parent of the householder’s sister or could be married to the parent of the householder’s sibling). Some datasets also include an IPUMS-generated variable called NSIBS, which reports the number of a person’s own siblings in the household (based on the IPUMS-derived family interrelationship variables; this is not reported by respondents in the original data).